4 Tablespoons butter, divided
2½ pounds medium yellow onions, thinly sliced
2 large portobello mushroom caps, diced
1 loaf French bread, torn into pieces
1½ cups shredded Gruyere cheese
1½ cups beef broth
½ cup freshly grated Parmesan cheese
Preheat oven to 350 degrees. Lightly butter a 9- by 13-inch baking dish and set aside. Melt 3 tablespoons of the butter in a large sauté pan over medium heat. Add the onions and cook slowly, stirring occasionally, until caramelized, about 30 to 40 minutes. Add the mushrooms and the remaining 1 tablespoon of butter and cook for 5 more minutes. Remove from heat. Line the bottom of the prepared baking dish with a thick layer of the bread (there may be some bread left over). Top with half of the onion-mushroom mixture and half of the Gruyere. Repeat the onion and cheese layers. Pour the beef broth evenly over the dish and top with the Parmesan. Bake for 35 to 40 minutes or until the top is bubbly and browned. Serves 8.