In a heavy large skillet over medium high heat, add 2 TB olive oil until hot. Add onions and stir until fragrant and browned, about 3 minutes. Add garlic and stir 1 minute.

Add shrimp and stir until they turn opaque; this should happen pretty quickly. Immediately remove from stovetop, cover to keep warm, and set aside.

Bring large heavy pot of well-salted water to boil. Add pasta and cook just until al dente, according to package instructions. Drain pasta, toss with 2 TB olive oil, cover to keep warm, and set aside.

Over low heat, melt the butter in the now-empty pasta pot. Add flour and whisk mixture constantly for at least 2 minutes or until it’s smooth.

Very slowly add the milk, half and half, and chicken broth. Turn heat to medium-low and continue to stir constantly until sauce is smooth and creamy, about 5 minutes. Take your time to ensure the sauce is not grainy; it should become thickened and smooth.

Stir in Parmesan and white cheddar cheeses, continuing to stir until cheeses are melted. Stir in sour cream until mixture is smooth. Taste and add salt and pepper, if needed.

Return cooked pasta to the pot and toss to coat. Gently stir in the shrimp, or serve shrimp on top of plated pasta. Garnish with chopped herbs of your choice. Sprinkle with extra parmesan cheese, if desired.