Combine the peeled shrimp with olive oil and Cajun seasoning in a medium bowl. Toss until each shrimp is evenly coated with the spice mixture. Set aside to marinate while you prepare the remaining ingredients.

Bring a large pot of lightly salted water to a rolling boil (about 3-5 minutes). Add the fettuccini and cook for 8 to 10 minutes until al dente. Drain well and set aside.

While pasta cooks, melt butter in a large frying pan over medium heat. Add the sliced mushrooms and chopped shallots, stirring frequently for about 5 minutes until the mushrooms release their moisture and turn golden brown. Transfer to a plate and set aside.

Add the seasoned shrimp to the same pan. Cook for 2 to 3 minutes, flipping once, until the shrimp turn pink with slightly charred edges and reach 145°F internal temperature. Remove from pan and set aside.

Pour 2/3 cup chicken broth into the pan and bring to a vigorous boil. Let it bubble uncovered for 2 to 3 minutes until reduced to about 1/4 cup. The liquid should thicken and coat the back of a spoon.

While the broth reduces, whisk together the sour cream and cornstarch in a small bowl until completely smooth. Stir in 1 cup of chicken broth until well combined. Pour this mixture into the reduced broth in the pan, stirring constantly. Cook until the sauce thickens and becomes bubbly (about 2-3 minutes), then let it simmer for 1 more minute.

Return the cooked shrimp, sautéed mushrooms, roasted red bell peppers, and capers to the pan. Stir everything together gently and heat through for about 2 minutes. Taste and season with salt and pepper as needed.

Divide the cooked fettuccini among 4 plates and top each with the shrimp mixture, making sure each portion gets plenty of sauce, shrimp, and vegetables.