A heavy-bottomed 3- or 4-quart saucepan, with a cover, to make the soup; small (11-ounce) soup crocks or a large crock or heavy casserole for the gratinee version.
Set the saucepan over medium-low heat and add the oil and butter. When the butter has melted, add the onions, thyme, and 1/2 teaspoon of salt and mix together thoroughly. Cover the pan and cook for about 10 minutes, stirring occasionally. When the onions are quite tender, uncover and raise the heat slightly. Cook for another 20 to 25 minutes, stirring frequently, until the onions are dark brown and have caramelized in the pan. Lower the heat if the onions are in danger of burning.
Stir in the hot stock, scraping any crystallized juices from the bottom of the pan, and bring the soup to boil. Taste and adjust the seasonings, adding salt and 1/4 teaspoon of black pepper or more to taste, and wine, if you like. The amount of salt will vary, depending on the broth. Cover and simmer for about 10 minutes.
The soup may be served plain or gratineed, as follows:
For onion soup gratinee
To make croutons, toast a dozen or so baguette slices on a baking sheet in a 400 degree F oven, until crisp and starting to color about 10 minutes.
When the soup is ready, arrange the individual crocks on the baking sheet. Put the croutons (whole or broken into large pieces) into the bottom of each crock, and sprinkle about 2 tablespoons of cheese on top. Ladle in a cup or more soup, to fill the crock to the inner rim (about 1/4 inch from the top). Heap a large mound of grated cheese all over the surface of the soup, using the rest of the cheese for each crock.
Place the baking sheet in the oven and bake for 30 - 40 minutes, until the cheese is dark golden brown and has formed crust over the soup. Move the hot crocks carefully onto individual plates and serve.
RECIPE CREDIT: Julia and Jacques Cooking at Home cookbook, published by Knopf.