I believe in diets based on moderation, with the occasional throw-caution-to-the wind splurge. And to revel in spring's vivid cornucopia of fruits without churning an egg-rich custard-based strawberry ice or chilling a creamy lemon mousse to crown ripe blueberries would be like eating tofu on Thanksgiving.

It is the rare occasion that warrants gustatory indiscretion like eggs Benedict. As luscious as runny poached eggs napped with buttery hollandaise (more eggs) on crusty English muffins with rich salty Canadian bacon may be, my intellect frequently wins out over my hedonistic desires.

I stop to factor in that there is almost a day's quota of cholesterol in a single egg yolk, and pause over the contamination fears of eggs not fully cooked.

But there are times I frankly don't care. I want to taste a rich custard or one of its derivatives, with all of its eggy goodness.

For years I thought to make a custard or mousse you had to be a magician. "Stir until thick enough to coat the back of a spoon" conjured up the foolish image of a wooden handle draped in a trench coat. "Do not allow it to boil or the eggs will curdle" was one of those caveats that caused ripples of anxiety detected by the Richter scale.

This need not be the case.

A custard is merely egg yolks mixed with sugar, perhaps a bit of salt, maybe some citrus juice or milk that is heated until thick. Once chilled, it can be used as is and would be called a cre`me anglaise in Paris and a custard sauce in Potomac. The thick emulsion also can be lightened with berry pure'e and enriched with whipped cream to form a mousse, or the same mixture can be frozen for homemade ice cream.

The Plot Thickens The reason this transformation occurs when egg yolks are gently heated is due to their chemical composition. The yolk, about one-third of an egg's edible weight, is about 50 percent water, 34 percent animal fat and other lipids (hence the cholesterol count), and 16 percent protein, with traces of glucose and minerals.

When egg yolks are heated, the egg proteins coagulate by bonding into a solid mass. If they are left undisturbed, as with a baked custard, they form a solid structure from what was once liquid. This is the custard of the tender Spanish flan or Creole rice pudding.

If you make the same mixture and keep stirring, it will become a creamy custard. The more liquid you add, the higher the temperature at which the thickening will start to occur.

Keep It Moving In old-fashioned cookbooks custards were always made in the top of a double boiler over simmering water. I don't have the patience. This method takes so long that eggs can hatch and mature into chickens before anything starts to resemble a custard, and yet you still can't leave it untended or the bottom will turn into scrambled eggs before the top thickens.

The secret of the properly thickened, satiny custard is to start by whisking the egg yolks in a heavy-bottomed saucepan until light colored. Then add the sugar, a tablespoon at a time, beating well between each addition. Beat this until it reaches the thickness of a pancake batter.

Now it's time to add the liquid, which thins out the mixture you've just spent time thickening. If the recipe calls for more than 1/2 cup of liquid, it will speed up the process if the liquid has been heated -- otherwise you'll be stirring all day. If it's a small amount, the temperature doesn't matter.

Place the pan over low heat, and trade your whisk for a wooden spoon. Stir continuously, and don't worry that it's about to be done until it's hot when you poke it and you see an occasional bit of steam rise.

At that time, look at the back of the spoon. If it is thickly coated, and if, when you draw a line through this coating, it remains visible rather than filling back in, then the custard is ready.

The Magic Number The best way to anticipate this moment is by taking the custard's temperature. If you remember 160 degrees, you can make a safe custard.

At 160 degrees there is no fear of salmonella bacteria, according to the Meat & Poultry Hot Line of the U.S. Dept. of Agriculture. And at that temperature you will have achieved the proper thickness, and the pot can be taken off the heat.

Damage Control So let's say the worst happens: Despite diligent stirring, you suddenly realize that there are lumps of cooked egg forming.

Throw it out? No.

Grab the pan off the burner and whisk it furiously to cool off the mixture and stop the proteins from bonding. Then, pour it slowly into a blender or a food processor fitted with the steel blade and just pure'e those little lumps away.

BLUEBERRIES TOPPED WITH LEMON MOUSSE (6 servings)

6 egg yolks

3/4 cup sugar

1/2 cup freshly squeezed lemon juice

2 teaspoons grated lemon zest

2 tablespoons Grand Marnier

1 1/2 cups whipping cream

4 cups fresh blueberries, rinsed and picked over

Beat the egg yolks with the sugar until thick and light colored. Beat in the lemon juice and lemon zest, and place the pan over low heat. Heat, stirring constantly, until the mixture is thick enough to coat the back of the spoon. Remove from the heat, beat in the Grand Marnier, and chill until cold.

Chill a mixing bowl and electric mixer beaters in the freezer. Beat the cream until stiff peaks form, and then fold the whipped cream into the custard mixture. Chill for up to 3 hours before serving.

To serve, divide the berries into small bowls, and top with a serving of the mousse.

Note: the custard can be made up to a day in advance and refrigerated, tightly covered with plastic wrap. However, do not whip the cream or fold in the mixture more than 3 hours in advance or the mixture may separate.

Per serving: 434 calories, 5 gm protein, 43 gm carbohydrates, 28 gm fat, 15 gm saturated fat, 354 mg cholesterol, 37 mg sodium.

-- Ellen Brown is a Washington-based food writer and prize-winning author of "The Gourmet Gazelle Cookbook."