In a medium bowl, combine shrimp, olive oil, and Cajun seasoning. Toss until evenly coated and set aside to marinate while prepping the other ingredients.
Bring a large pot of salted water to a boil. Add fettuccine and cook for 8–10 minutes, until al dente. Drain and set aside.
In a large frying pan, melt butter over medium heat. Add mushrooms and shallots, cooking until tender and golden brown. Remove from the pan and set aside.
In the same pan, add the seasoned shrimp. Cook for 2–3 minutes, turning once, until pink and opaque. Remove and set aside.
Add ⅔ cup chicken broth to the pan and bring to a boil. Let it reduce to ¼ cup (about 2–3 minutes).
In a small bowl, whisk together sour cream, cornstarch, and 1 cup chicken broth. Pour into the pan and stir until thickened and bubbly. Simmer for one more minute.
Return shrimp, sautéed vegetables, roasted red bell peppers, and capers to the sauce. Stir to combine and heat through.
Toss the cooked fettuccine with the sauce, coating evenly. Serve immediately, garnished with fresh herbs or extra Cajun seasoning if desired.
