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. Remove sausage from casings; discard casings.

Drizzle pepper halves with oil, salt, and pepper; place cut sides down on a lightly oiled baking sheet. Place whole garlic cloves on a 6-x-6-inch piece of aluminum foil and drizzle with oil, salt, and pepper. Cinch foil to make a closed pouch; place on baking sheet. Roast until pepper begins to char and garlic is soft, 20-25 minutes. Remove from oven.

While veggies roast, add gemelli to boiling water. Cook until al dente, 9-11 minutes. Reserve 3/4 cup pasta water, 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, 6-8 minutes. Pour out any excess fat from pan. If desired, stir in a pinch of chili; cook 15 seconds. Add tomato paste and 1/2 cup reserved pasta cooking water. Simmer until thickened, about 2 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 smash with a fork. Return pan with sauce to low heat. Add garlic and stir to combine. Stir in sliced bell pepper, pasta, sour cream, 2 TBSP butter, and half the Parmesan. (TIP: If sauce seems dry, stir in a splash of remaining reserved cooking water.) Season with salt and pepper.

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