How to Make It
Cook pasta according to package directions. Drain and set aside.
Preheat oven to 450°F. Place tomato slices on a wire rack on a baking sheet. Brush on oil, and sprinkle with oregano, pepper, and 1/4 teaspoon of the salt. Bake in preheated oven until slightly charred, about 18 to 20 minutes.
Meanwhile, melt butter in a medium saucepan over medium until melted, about 1 minute. Whisk in flour, and cook, whisking constantly, until the mixture takes on a slightly nutty color and smell, about 2 to 3 minutes. Whisk in dry mustard. Slowly whisk in milk, whisking constantly, and continue to cook until slightly thickened, about 4 to 5 minutes. Remove from heat, and cool slightly, about 3 minutes. Slowly whisk in Monterey Jack, Cheddar, fontina, and remaining 1 teaspoon salt; whisk constantly to combine and melt cheeses, about 2 minutes. Stir in pasta.
Spoon pasta mixture into a lightly greased 11- x 7-inch (2-quart) baking dish, and top with tomato slices. Sprinkle with Parmesan cheese. Bake at 450°F until bubbly and tomatoes are charred and aromatic, about 10 minutes. Serve immediately.