In a bowl, toss the shrimp with 2 teaspoons Cajun seasoning and 1/4 teaspoon black pepper until evenly coated. Set aside. Slice the beef sausage into rounds, dice the onion, and mince the garlic. Bring a large pot of salted water to a boil, add the fettuccine, and cook according to package directions until al dente. Drain and set aside.

Heat a large skillet or sauté pan over medium-high heat and add 1 tablespoon of olive oil. Add the seasoned shrimp and cook for 2 to 4 minutes, flipping halfway through, until pink and cooked through. Transfer to a plate. Add the sliced sausage to the pan (add a drizzle more oil if needed) and cook until browned and caramelized on both sides, about 4 to 5 minutes. Transfer to the plate with the shrimp.

If needed, add another drizzle of olive oil to the pan. Sauté the onion, garlic, and thyme sprigs until fragrant and softened, about 2 minutes. Pour in the chicken broth, heavy cream, crushed tomatoes, and Worcestershire sauce. Stir to combine, scraping up any browned bits from the bottom. Bring to a simmer.

Let the sauce simmer gently. Stir in the remaining 1 tablespoon Cajun seasoning (start with less and add to taste), sugar, garlic powder, onion powder, dried oregano, cayenne pepper (if using), smoked paprika, and black pepper. Mix well and let simmer over medium heat for 5 minutes.

Add the grated Parmesan cheese to the sauce and stir occasionally for another 5 minutes. Add the cooked pasta and browned sausage, tossing to coat and heat through, about 2 to 3 minutes. Gently fold in the cooked shrimp, then remove from heat. Garnish with fresh parsley, extra cracked black pepper, and more Parmesan if desired. Serve immediately.