Bring a large pot of salted water to a boil. Cook the fettuccine according to package directions until al dente, drain, and set aside.
Heat the olive oil in a large high-sided skillet over medium-high heat.
Toss the shrimp with the Cajun seasoning, then add to the hot skillet and cook about 4–5 minutes until pink and just cooked through. Remove the shrimp to a plate and set aside.
In the same skillet, add the sliced Andouille sausage and diced onion; sauté until the onion is translucent and the sausage begins to brown, about 4–5 minutes.
Add the minced garlic and cook about 30 seconds until fragrant.
Pour in the chicken stock and scrape up any browned bits from the bottom of the pan.
Stir in the heavy cream, grated Parmesan, black pepper, and cayenne; simmer over medium heat for about 5 minutes until the sauce thickens, stirring frequently to prevent scorching.
Add the cooked pasta and reserved shrimp to the skillet and toss gently to coat in the sauce. If needed, thin the sauce with a little milk or let it reduce to reach desired consistency.
Serve immediately with a sprinkle of minced parsley, if desired.