Bring a large pot of salted water to a boil, cook the fettuccine according to package directions until al dente, then 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 shrimp 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 for 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, stirring frequently, until the sauce thickens slightly.
Add the cooked pasta and reserved shrimp to the skillet and toss gently to coat; if the sauce is too thick, thin with a splash of milk, or simmer a bit longer if too thin.
Serve immediately with a sprinkle of minced parsley, if desired.
