In a medium bowl, toss the peeled and deveined shrimp with olive oil and Cajun seasoning until each shrimp is well-coated. Allow the shrimp to marinate for at least 15 minutes.

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

In a large skillet, melt the butter over medium heat. Add the sliced mushrooms and shallots, sauté for 5-7 minutes until tender and golden.

Increase heat to medium-high and add the marinated shrimp. Cook for 2-3 minutes until pink and opaque, then remove from skillet.

Pour in chicken broth and bring to a boil. Reduce to about a quarter of its volume (4-5 minutes).

Mix sour cream and cornstarch, stir in a cup of broth. Gradually add this mixture to the skillet, stirring until thickened (2-3 minutes).

Return sautéed mushrooms, shrimp, roasted red peppers, and capers to the skillet. Cook for 2-3 minutes until heated.

Plate cooked fettuccini and spoon the creamy shrimp and sauce over the top. Garnish with fresh herbs if desired.