Call them the Pop Tarts of the '80s. Mix-in-the-bowl flavored hot cereals have become the fast-food generation's quick at-home breakfast. While certainly not new, this genre of hot cereal fits today's marketplace, one where speedy, nutritious-sounding fare is dangled in front of health-conscious people on the run.

Instant products in such flavors as raisin spice, honey graham or apples and cinnamon are reviving the hot-cereal market. According to Advertising Age magazine, the $420-million hot-cereal category has grown 10 percent in dollar volume over the past year, due to the "more expensive cook-in-bowl products." Last year was the most successful year for Quaker Oats, which leads the instant segment, according to Advertising Age.

This year Quaker Oats introduced two new flavors to its 10-flavor line -- strawberries 'n cream and raisins, dates and walnuts. Ralston Purina has just introduced a line of instant cereals under the Sun Maid label and in 1982 Nabisco reformulated its Mix 'n Eat Cream of Wheat and added four new flavors.

The cereals, say company spokespeople, are targeted either to the busy professional who has neither the time nor inclination to prepare a steamy pot of oat groats, or to children, as a cold-weather alternative to sugary cold cereals (flavors such as strawberries 'n cream or hot chocolate indeed have the same ring as cold cereals such as Strawberry Shortcake or Cocoa Puffs).

In fact, like many cold cereals, the majority of the instant flavored hot cereals list sugar as the second most dominant ingredient; some even contain two kinds of sugar (sugar and corn syrup, for example). In addition, some contain artificial flavors and all list some kind of preservative.

The fact remains, however, that you can still buy unsweetened, fiber-packed cereals in supermarkets in either the natural food section or in the regular hot-cereal section. (Quaker Oats, for example, has a relatively new product marketed under the "Mother's" label in Oat Bran, Oat and Whole Wheat, and there are a variety of other plain, packaged whole-wheat cereals or whole-grain cereal mixtures.)

In addition, there are a host of grains available in bulk at health food stores -- without the preservatives, the artificial flavors and the fancy packaging. Here then, are some suggestions beyond Maypo on how to make your own flavored hot cereal -- the "old-fashioned" way. It's a cold-weather comfort food that should never go out of style. Grains

For those cooking with bulk grains, the following are some hints on cooking times and procedures. Remember that instead of using water to cook hot cereal, you can substitute half or all of the liquid with milk, preferably skim. (This will make the cereal creamier in taste and texture.) Figure about 1 cup of raw cereal for 4 generous servings.

Oatmeal: The difference between the various kinds of oatmeal and their cooking times has to do with the size and way they are cut.

Oat groats are the whole, hulled grain. Simmer 1 cup whole groats in 2 cups water or milk in a covered saucepan for about 1 hour.

Steel-cut oats are oat groats that have been sliced thinly lengthwise. Cook 1 cup steel-cut oats in 3 cups water for 45 minutes.

Rolled oats come in three varieties: old-fashioned, quick-cooking and instant. The old-fashioned oats are made by husking, toasting, steaming and rolling oat groats; they take five minutes to cook. Quick-cooking oats are made from groats that have been cut into pieces before they are rolled, producing a thinner flake and thus reducing the cooking time of the old-fashioned variety by three or four minutes. Instant oatmeal, or the mix-in-the-bowl variety, is rolled from groats cut into even thinner pieces. They are precooked, dried and processed with salt.

According to the Quaker Oats Co., the three types of rolled oats do not differ nutritionally; according to Ruth Matthews of the Nutrient Data Research Branch of the USDA, all kinds of oatmeal are excellent sources of protein and fiber.

Wheat: Besides wheat berries, which take a long time to cook (2 or 3 hours), try cracked wheat. Bring 2 1/2 cups water to a boil. Stir in 1 cup cracked wheat, cover and simmer about 25 minutes.

Buckwheat: Buckwheat groats, or kasha, can be eaten as a breakfast cereal. Bring 2 cups salted boiling water to a boil, stir in 1 cup buckwheat groats and simmer, covered, about 20-30 minutes, or until all water is absorbed.

Millet: For 1 cup whole millet, use 4 cups boiling water. Lower heat and simmer, covered, for about 30 minutes. Millet is especially good cooked with milk.

Flake cereals: The following cooking times are for 1 cup of cereal and 2 1/2 cups of water. Rice flakes: 3 to 5 minutes. Barley flakes: 6 minutes. Wheat flakes: 8 minutes. Rye flakes: 16 to 20 minutes.

Mixtures: Mix together anywhere from 3 to 7 different kinds of cereal in one pot. Thinners

Similar to pizza eaters who are divided about the virtues of thin versus thick crusts, hot-cereal eaters differ about how thick or thin the cereal should be, as well as whether milk or cream should be poured around the rim of the cereal bowl. So for those in favor of "the rim," here are a few alternatives, enough for 1 to 2 servings:

* Make a warm apple butter sauce by combining 1/4 cup unsweetened apple butter in a saucepan with 1/4 cup water. Stir and heat until warm.

* In a small saucepan, heat 1/4 cup nut butter such as cashew or almond with 6 tablespoons water. Stir into cereal.

Make fruit syrup by using unsweetened frozen raspberries, blueberries or strawberries. Thaw and pure'e in a food processor or blender.

* Add maple syrup or honey to taste to 1/4 cup yogurt. Or just go with plain yogurt or, alternately, simply a drizzle of syrup or honey. Toppings

Dried fruit: Soak dried apricots, golden or dark raisins, apples, dates or currants in fruit juice for 5 minutes. Add to cereal in last few minutes of cooking. Or just sprinkle dried fruit on top of cooked cereal.

Fresh fruit: Sliced pears, bananas and apples are obvious choices; pears and apples can even be cooked with the cereal. Also top cereal with sectioned tangerines, pomegranate seeds, or for a particularly trendy breakfast, sliced kiwi.

Make curried apples by saute'ing sliced apples in butter over high heat for about 10 minutes. Season with curry powder as apples cook. Toss with cereal, adding raisins if you wish.

Nuts and seeds: Aside from chopped cashews or walnuts, sprinkle finished cereal with sunflower seeds, bits of chopped macadamia nuts, filberts or pine nuts. In the seed category, try sesame seeds, poppy seeds or pumpkin seeds.

Make your own dried fruit and nut mix, or sprinkle cereal with granola.

Cheese: With nuttier type cereals (bran or buckwheat), grate a tablespoon or two of mild cheese (such as monterey jack or a mild cheddar), sprinkle on cooked, hot cereal and stir until cheese is melted.

Spices and sprinkling: Season cereal with freshly grated nutmeg, cinnamon or cloves. Add a few drops of vanilla or almond extract, or sprinkle cereal with flaked coconut or wheat germ.