Adjust rack to top position and preheat oven to 450 degrees. Bring a large pot of salted water to a boil. Wash and dry all produce. Halve, peel, and thinly slice onion. Trim and halve zucchini lengthwise; cut crosswise into 1/2-inch-thick half-moons.

Toss zucchini on a baking sheet with a drizzle of oil, half the Italian Seasoning (you’ll use the rest later), and a pinch of salt and pepper. Roast on top rack, tossing halfway through, until browned and tender, 14-16 minutes. Tent with foil to keep warm.

Once water is boiling, add cavatappi to pot. Cook until al dente, 9-11 minutes. Reserve 11/2 cups pasta cooking water (2 cups for 4 servings), then drain.

Heat a drizzle of oil in a large pan over medium-high heat. Add onion and a pinch of salt and pepper. Cook, stirring occasionally, until softened, 5-6 minutes. Add sausage and cook, breaking up meat into pieces, until browned all over but not yet cooked through, 3-4 minutes (it’ll finish cooking in the next step).

Add tomato paste and remaining Italian Seasoning to pan. Cook, stirring constantly, until fragrant, 1-2 minutes. Stir in stock concentrate, 1 cup reserved pasta cooking water (11/2 cups for 4 servings), and a big pinch of salt and pepper. Bring to a simmer; cook until sauce has slightly thickened and sausage is cooked through, 2-3 minutes. Reduce heat under pan to medium low.

Stir cavatappi, zucchini, sour cream, and 1 TBSP butter (2 TBSP for 4 servings) into pan until thoroughly combined. (TIP: If needed, stir in more reserved cooking water a splash at a time until pasta is thoroughly coated in sauce.) Divide between bowls. Sprinkle with Parmesan and serve.