CHEF LOUIS Szathmary, author and chef-proprietor of the Bakery restaurant in Chicago, whisked into town last week, and was asked about cooking trends. Trends are just the frosting on the layer cake, he answered. And the layers? Year in and year out, 52 percent of his restaurant's customers order beef wellington. Still. But what is the most stubbornly unchanging of all eating habits? He is never able to budge his customers from eating their salad before the main course, he insisted. The U. S. is in the clutches of California.

This time of year our thoughts turn to tomatoes with fresh basil. And though it is going to be months yet before we can fulfill those fantasies, we can get started laying the groundwork now. We can begin with an herb catalog from Earthworks Herb Garden Nursery, 923 N. Ivy St., Arlington, Va. 22201. 243-2498. The eight-page catalog is free, full of explanations--for Japanese parsley and New Jersey tea, for instance--with recipes, and a chart outlining growing conditions for six dozen herbs. We're beginning to dream of fresh-caught rockfish and grills lined with rosemary branches.

It probably won't do much for the fit of your designer jeans, but now you can buy designer sardines. They're from Maxim's de Paris, being sold at Dolly Kay boutique. Delicious fat little fish they are, either with truffles, for $6 a 4-ounce tin; or dated and labeled as "vintage" sardines (1981 was apparently a good year, for these are mellow and rich) for $6.50, said to be at their best after they have aged for three years. Dolly Kay has a whole line of Maxim's fashionable edibles, but the sardines were more our style than the oils, vinegars, cookies, teas or quail eggs. In addition to the sardines, the ready-to-freeze jar of raspberry sherbet at $8.50 is worthy of a three-star restaurant. If your taste grows to grand enough proportions, Dolly Kay will pack them in a Maxim's hatbox.

We have also lately been mildly obsessed with Rachel's Brownies, individually wrapped and sold at speciality shops for about 75 cents. They're a cottage industry, the cottage being in Malvern Pennsylvania, and said to handmade with no preservatives. While we wonder that Rachel chooses to make her best-of-everything brownies with margarine instead of butter, we freely admit that they are outstanding brownies, so moist that they teeter on the edge of fudge, so chocolatey and dense that one wouldn't dream of eating more than one--unless another one just happened to be handy.

People who plan ahead might start next Thursday on Thanksgiving. That's when Beverly Cox, ex-Washingtonian and author of "Cooking Technique" will be demonstrating how to prepare a turkey for a buffet, at 7:30 to 9 p.m. in the Seven-Corners branch of Kitchen Bazaar. She will show how to bone the breast before roasting, then slice the cooked breast and replace it so the turkey looks whole again. Neat trick. And it might convince you to serve turkey for St. Patrick's Day. Cox will also be demonstrating March 13 at the Connecticut Avenue branch of Kitchen Bazaar, from 1 to 4 p.m.

As for Greek Easter, there is not a moment to lose. Tomorrow at Williams-Sonoma, from 1 p.m. to 2 p.m., Rosemary Hinton, who has a cooking school in Crete, will be demonstrating Greek cooking with special emphasis on dishes for the April 18 Greek Easter.