I cannot find any "Marinated Tomatoes and Onions" recipe in either of her books, but she has a "Fancy Sliced Tomatoes" recipe that sounds like what you are looking for:

"Fix this a few hours ahead if you can, so the flavor has a chance to burgeon.

tomatoes
onions
salt
pepper
dried basil
sugar
vinegar
olive oil

Put a layer of sliced unpeeled tomatoes in a pretty shallow bowl about eight or ten inches in diameter, and put a layer of sliced onions (bermuda, green or what have you) on top of it. Sprinkle a bit of salt and pepper around, a pinch of basil, a half a teaspoon of sugar, and one teaspoon each of vinegar and olive oil. Add another layer of tomatoes and onion slices and repeat the seasonings. Keep going in this fashion, depending on how many people you are serving and how tired you get."

Let us know if this is it!