Wash and dry all produce. Bring a large pot of salted water to a boil. Halve, peel, and dice onion. Mince or grate garlic. Finely chop chives. Trim, then thinly slice mushrooms. Trim woody bottom ends from asparagus, then cut into 1-inch pieces. Core, seed, and remove white ribs from bell pepper, then cut into 1-inch squares.

Fill a large bowl with ice water. Once water in pot is boiling, add asparagus. Cook until bright green but still crisp, 2-3 minutes. Using a slotted spoon, transfer to bowl with ice water. Add orecchiette to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. Reserve 1/2 cup pasta cooking water, then drain.

Heat a drizzle of olive oil in a large pan over medium heat. Add mushrooms and cook, tossing, until tender and a few shades darker, 4-5 minutes. Season with salt and pepper. Remove from pan and set aside.

Heat a drizzle of olive oil in same pan over medium heat. Add onion and bell pepper. Cook, tossing, until lightly browned, 6-8 minutes. Stir in garlic and a pinch of chili flakes (to taste). Cook until fragrant, about 30 seconds. Season with salt and pepper.

Drain ice water from asparagus. Add to pan with orecchiette, mushrooms, sour cream, Parmesan, half the chives, and a splash of reserved pasta cooking water to pan. Toss until well combined and a thick sauce has formed, 1-2 minutes. Season with salt and pepper. TIP: Add more pasta cooking water to loosen if sauce is thick.

Divide pasta mixture between plates. Garnish with remaining chives and serve.