Preheat the oven to 350 degrees F.
In a large skillet coated with olive oil, sauté the onion over medium heat until it softens, 2-3 minutes. Add the zucchini and continue to cook, stirring occasionally, until the zucchini is crisp tender, about 4-5 minutes more. Remove to a large bowl.
In the same skillet, brown the sausage, crumbling as you go. Remove to the bowl with the zucchini and onions.
To keep things easy, I make the white sauce in the same skillet: Melt the butter over medium heat. Add the flour and stir until the butter and flour are well combined. Pour in milk, stirring constantly as it thickens. Add the salt and pepper and 1/2 cup of the cheese, stirring until just melted. Remove from the heat.
Combine the cheese sauce with the sausage, zucchini mixture, tomatoes, and cooked pasta. Transfer the mixture to a lightly-greased, 2-quart baking dish. Top with the remaining 1/2 cup of cheese and bake for 15-20 minutes or until the mixture is hot throughout and the cheese is melted.
Garnish with fresh basil or parsley, if desired, and serve hot.
