Preheat the oven to 425 degrees.
Heat the olive oil in a large nonstick skillet over moderate heat and stir in the onion. Cook, stirring, for 2 minutes. Add the garlic and cook, stirring, for 1 minute. Raise the heat slightly and add the bell peppers. Cook, stirring for 2 minutes. Add the zucchini and yellow squash and cook, stirring for 2 minutes. Season with salt and pepper and set aside.
Cook the pasta in a large kettle of boiling salted water for 4 minutes. Drain the pasta and rinse it with cold water.
Heat the butter in a medium-size heavy saucepan over moderate to moderately high heat until it becomes a medium nutty brown, being careful not to burn it. Stir in the flour, rosemary, sage and bay leaf. Cook, stirring constantly, for 2 minutes. Slowly add the milk and cook, stirring, for 4 to 5 minutes, or until sauce is very thick--the consistency of sour cream. Strain the sauce and season well with salt and pepper. Cool slightly.
Return the pasta to the same large kettle and stir in the sauce. Add the vegetables and 1 cup grated fontina and blend completely. Place the mixture in a deep earthenware or glass baking dish (about 9 inches by 13 inches by 2 inches). Sprinkle with the remaining fontina. Bake for 20 minutes, until the pasta is brown and crunchy on top.
Suggested drink: Sauvignon, Castello della Sala, Umbria