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 onions. Trim and halve zucchini lengthwise; cut crosswise into 1/2-inch-thick half-moons.

Toss zucchini on a baking sheet with a large drizzle of oil, 2 tsp Tuscan Heat Spice, and a pinch of salt and pepper. Roast on top rack, tossing halfway through, until browned and tender, about 15 minutes. Loosely cover with foil to keep warm.

Once water is boiling, add gemelli to pot. Cook until al dente, 9-11 minutes. Reserve 2 cups cooking water, then drain.

Heat a drizzle of oil in a large pan over medium-high heat. Add onions 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, 3-4 minutes.

Add tomato paste and remaining Tuscan Heat Spice to pan. Cook, stirring constantly, until fragrant, about 1 minute. Stir in stock concentrates, 11/2 cups reserved pasta cooking water, and a large pinch of salt and pepper. Bring to a gentle simmer and cook until sausage is cooked through and sauce has thickened slightly, 2-3 minutes.

Reduce heat under pan to medium low. Stir in pasta, zucchini, sour cream, and 1 TBSP butter until thoroughly combined. (TIP: If pan seems dry, stir in remaining cooking water, a splash at a time, until mixture is creamy.) Divide pasta between bowls. Sprinkle with Parmesan.