Preheat oven to 350 degrees F. In a skillet melt 3 tablespoons of butter. Add shallot and cook, stirring occasionally for 3 minutes, or until softened. Add the mushrooms, salt and pepper, thyme, and rosemary. Cook, stirring occasionally for 2 minutes more or until the mushrooms are lightly golden. Transfer mixture to a bowl and stir in chives or parsley and set aside. Now make the sauce. In a saucepan set over moderately low heat melt butter, add the flour and cook the mixture, whisking, for 3 minutes. Add milk, bring mixture to a simmer, whisking, and simmer, stirring occasionally for 5 minutes or until lightly thickened. Remove saucepan from the heat and stir in the fontina, mozzarella, and gorgonzola and 2/3 cup of the Parmesan cheese, a little at a time, until well combined. Season to taste with salt and pepper. To assemble the gratin; in a large bowl toss the pasta with the mushroom mixture. Add 3/4 of the sauce and stir gently to combine. Using remaining 3 tablespoons of butter, butter a 2 to 3 quart gratin dish arrange pasta and diagonal rows of the tomatoes. Top evenly with remaining sauce. Place in oven and bake for 25 minutes or until bubbling and golden.
Recipe courtesy of Michele Urvater