Toast the baguette slices and let them cool. Spread a generous layer of butter on each slice (you will need about 5 tablespoons), then lay the slices close together on a baking sheet and top with all but 1/2 cup of cheese. In a large saucepan, melt the remaining 4 tablespoons butter over medium heat. Add the onions, season with salt and sauté, stirring occasionally, until very soft and golden, about 15 minutes.
In a 5-quart casserole, arrange a layer of bread slices (about 1/3 of them). Spread 1/3 of the onions on top, followed by 1/3 of the tomato purée. Repeat for two more layers. Sprinkle with the remaining 1/2 cup cheese. To avoid boiling over, the casserole must not be more than 2/3 full. In a saucepan, bring 1 1/2 quarts water to a boil. Add the salt. Very slowly pour the salted water into the casserole, near the edge, so that the liquid rises just to the top layer of cheese without covering it. (Depending on the size of your casserole, you may need more or less water.)
Preheat the oven to 350 degrees. Put the casserole on the stove and simmer uncovered for 30 minutes, then transfer to the oven and bake uncovered for 1 hour. The soup is ready when the surface looks like a crusty, golden cake and the inside is unctuous and so well blended that it is impossible to discern either cheese or onion. Each person is served some of the baked crust and some of the inside, which should be thick but not completely without liquid. Serves 6.
- 1 baguette, cut into 1/2-inch slices (about 25 to 30)
- 9 tablespoons butter, softened
- 9 ounces Emmental cheese, finely grated
- 8 medium yellow onions, thinly sliced (about 12 cups)
- 1 tablespoon kosher salt, more to taste
- 1 cup tomato purée.