In a bowl, combine peeled and deveined shrimp with olive oil and Cajun seasoning. Let shrimp marinate for about 15 minutes at room temperature.

Bring a large pot of salted water to a boil. Add fettuccini and cook according to package instructions for 8-10 minutes until al dente. Drain and set aside.

In a large skillet, melt 2 tablespoons of butter. Add sliced mushrooms and minced shallots, sauté for about 5-7 minutes until softened and slightly golden.

In the same skillet, add the marinated shrimp in a single layer and sear for about 2-3 minutes per side until cooked. Remove from skillet.

Pour in 1 cup of chicken broth and bring to a boil. Let it simmer until it thickens slightly, about 5 minutes. Stir in sour cream and dissolved cornstarch until thick and bubbly.

Return shrimp, sautéed vegetables, sliced roasted red bell peppers, and capers to the skillet with the sauce. Stir gently until everything is well coated and warm, adjusting seasoning with salt and pepper.

To plate, create a nest of fettuccini on each plate, ladle the shrimp mixture over the top, and serve immediately.