Adjust rack to middle position and preheat oven to 400 degrees. Bring a large pot of salted water to a boil. Wash and dry all produce. Halve bell pepper, then remove and discard core and stem. Thinly slice chili, removing seeds for less heat. Remove sausage from casings; discard casings.

Drizzle bell pepper halves with oil, salt, and pepper; place cut sides down on a lightly oiled baking sheet. Place garlic cloves on a small piece of foil and drizzle with oil, salt, and pepper; cinch into a packet and place on baking sheet. Roast until bell pepper begins to char and garlic softens, 20-25 minutes.

Once water is boiling, add gemelli to pot. Cook until al dente, 9-11 minutes. Reserve 3/4 cup pasta cooking water (11/2 cups for 4 servings), then drain.

Heat a drizzle of olive oil in a large pan over medium-high heat. Add sausage and cook, breaking up meat into pieces, until browned and cooked through, 6-8 minutes. Pour out and discard any excess fat from pan. If desired, stir in a pinch of chili; cook until fragrant, 15 seconds. Add tomato paste and 1/2 cup reserved pasta cooking water (3/4 cup for 4 servings). Simmer until thickened, 2-3 minutes. Turn off heat.

Transfer roasted bell pepper halves to a cutting board; thinly slice. Remove roasted garlic cloves from foil; transfer to cutting board and gently mash with a fork. Return pan with sauce to low heat. Add mashed garlic and stir to combine. Stir in sliced bell pepper, gemelli, sour cream, 2 TBSP butter (4 TBSP for 4 servings), and half the Parmesan. (TIP: If sauce seems dry, stir in a splash of remaining reserved pasta cooking water.) Season with salt and pepper.

Divide pasta between bowls. Sprinkle with remaining Parmesan and, if desired, a pinch of remaining chili.