How to Make It
In a medium frying pan, melt one tablespoon of the butter over moderately low heat. Add the bell peppers and 1/4 teaspoon of the salt and cook until soft, about 10 minutes.
Heat the broiler. Butter a large shallow baking dish.
In a large pot of boiling, salted water, cook the fusilli until just done, about 13 minutes. Drain and return to the hot pot. Toss the pasta with the remaining 3 tablespoons butter and 3/4 teaspoon salt, the sautéed bell peppers, the fontina, mozzarella, 3 tablespoons of the Parmesan, and the black pepper.
Transfer the pasta to the prepared baking dish and sprinkle the remaining 3 tablespoons Parmesan over the top. Broil until the top starts to brown, 3 to 4 minutes. Serve with additional Parmesan.
Browned and Bubbling
Cheese often turns stringy when tossed with hot pasta. Broiling the dish after tossing melts the strings into a smooth sauce. An added bonus: that irresistible brown crust.