Stuffed Onions Baked in Cream Sauce

The tomato purée, used in the sauce, is best prepared in summer from garden-ripe tomatoes. Coarsely chop them, then crush in a saucepan with a chopped onion, crushed garlic clove, a pinch of herbs and salt. Boil for 10 or 15 minutes and pass through a sieve. The heavy purée will rapidly settle to the bottom of the bowl, leaving a topaz-tinted liquid on the surface, which can be ladled off (but not discarded; chilled, it is a delicious summer drink). The remaining purée may still have to be