Celebrating Thanksgiving in London

Elizabeth Lambert Ortiz, an Englishwoman married to a Mexican diplomat with a ''Texas'' accent who lives in London, has just produced a remarkable book, Cooking with the Young Chefs of France (M. Evans and Co., December publication, you may ask, cosmopolitan?

And the answer is yes. Mrs. Ortiz likes to eat, likes to cook, and likes to write and talk about it. You see it the minute you meet the dynamic, witty woman who has produced some of the most original, and practical, cookery books on Mexican, Caribbean, and Japanese food.

Mrs. Ortiz' book on French chefs emerged as a result of years of research and travel throughout the country. It was with a growing fascination that she discovered what she saw as deep, real and lasting changes in French cooking, the sort that comes along every 100 years or so. ''Changes which,'' she remarked with good humor over lunch at London's elegant Berkeley hotel,''have also produced a lot of nonsense in the name of nouvelle cuisine.''

Mrs. Ortiz' book, however, is anything but nonsense. It is a wide-ranging collection of recipes that include marvelous salads of everything from shrimp, artichoke heart, and avocado, Salade Chaubuisson, to a salad of spring vegetables with smoked salmon, Salade de Petits Legumes Au Saumon Fume.

In spite of her travels - and her own profound Englishness - Mrs. Ortiz considers Thanksgiving the great and festive holiday.

There is, of course, a Norfolk turkey at the top of Mrs. Ortiz' shopping list , this from her butcher, one Mr. Morse of Ealing whose shop is below the flat where Thomas Huxley was born.

For soup: a clear fish fume with whole clams in it because it reminds her of New England and happy holidays there. She gets either tinned clams, or if she can, the Pallorde variety from France often available at Harrods.

For succotash, there are limas, again, at Harrods, or frozen limas at Safeway and corn is now almost universally available in England. The oysters would come from her fishmonger, a Cypriot who runs Scottish Fisheries in Northfields, and the yams and pumpkins from the rambling outdoor Shepherd's Bush market in the West of London.

The recipes Mrs. Ortiz uses are the simplest, most traditional: a classic oyster stuffing for the turkey unless oysters are ''too dear'' in which case one made of chestnut and sausage. For dessert as an alternative to pumpkin pie, Mrs. Ortiz has concocted a cranberry vacherin. Cranberry Vacherin Ortiz

Cook cranberries in sweetened orange juice. When they pop, drain and puree them, then fold into whipped cream and spoon into a vacherin shell.