Preheat the oven to 400 degrees F. Butter a 9x9-inch baking or casserole pan.
Heat the olive oil in a medium-sized frying pan over medium heat and cook the onion, bell pepper and garlic with a couple dashes each of salt and pepper until softened about 5 minutes, stirring often.
Melt in the butter then stir in the flour and cook 2 - 3 minutes stirring often.
Slowly stir in the milk. Turn the heat up slightly and bring to a simmer. Simmer 3 - 4 minutes, stirring often.
Stir in the Italian seasoning, Parmesan cheese and ½ teaspoon salt plus ¼ teaspoon black pepper. Remove from the heat. Stir in the lemon zest.
Stir the pasta, the sauce, 1 cup of the ricotta, the mozzarella and zucchini together.
Spoon it into the casserole dish and dollop spoonfuls of the remaining ricotta on top.
Bake 25 minutes. Serve with more Parmesan cheese on top.