Before starting, preheat broiler to high and wash and dry all produce. Add 10 cups water and 2 tsp salt in a large pot (NOTE: Use same for 4 ppl). Cover and bring to a boil over high heat. Add rigatoni to boiling water. Cook, stirring occasionally, until tender, 9-10 min. When rigatoni is done, reserve 1/2 cup pasta water (dbl for 4 ppl), then drain and set aside.

While rigatoni cooks, cut zucchini into ½-inch pieces. Roughly chop parsley.

Heat a large oven-proof pan over medium-high heat. When hot, add 1 tbsp oil (dbl for 4 ppl), then zucchini, half the onion and half the Italian seasoning. Cook, stirring often, until veggies are tender-crisp, 3-4 min. Season with salt and pepper. Remove pan from heat then transfer veggies to a plate.

Heat the same pan over medium-high heat. When hot, add ½ tbsp oil (dbl for 4ppl), then sausage. Cook, breaking up the sausage into smaller pieces, until no pink remains, 4-5 min.** Add remaining Italian seasoning. Cook, stirring often, until fragrant, 1 min. Add crushed tomatoes and 1 tsp sugar (dbl for 4ppl). Reduce heat to medium and cook, stirring occasionally, until sauce is slightly thickened, 5-7 min.

While tomato sauce cooks, heat the same pot (from step 1) over medium heat. When hot, add 1 tbsp butter (dbl for 4 ppl), then remaining onions. Cook, stirring often, until slightly softened, 2-3 min. Sprinkle over flour. Cook, stirring often, until coated, 1 min. Add 1/2 cup milk (dbl for 4 ppl) and cook, stirring often, until thickened, 1 min. Remove pot from heat, then stir in sour cream. Season with salt and pepper.

Add rigatoni, reserved pasta water, veggies and half the parsley to pan with tomato sauce. Toss to combine. (NOTE: For 4 ppl, transfer mixture to a lightly-oiled 9x13-inch baking dish.) Spread cream sauce on top of pasta, then sprinkle over Parmesan. Broil in the middle of the oven, until golden brown, 3-4 min. Divide Pastitsio between plates. Sprinkle over remaining parsley.