Bring a large pot of salted water to a boil. Add the fettuccine and cook according to the package instructions until al dente. Reserve ½ cup of pasta water, then drain the pasta in a colander and set aside.
In a large skillet, heat the butter and olive oil over medium heat. Add the chopped onion and bell pepper, sautéing for about 3-4 minutes until softened.
Add the minced garlic to the skillet and sauté for an additional 1-2 minutes until fragrant.
Pour in the heavy cream and chicken broth, stirring well to combine. Bring the mixture to a gentle simmer.
Stir in the Cajun seasoning, salt, black pepper, and cayenne pepper (if using). Let the sauce simmer for about 5 minutes to thicken.
Add the shrimp to the skillet, cooking for about 3-4 minutes until they turn pink and are cooked through.
Gently fold in the lump crab meat and cooked fettuccine, adding a bit of the reserved pasta water if the sauce needs to be loosened up.
Cook everything together for an additional 2 minutes, ensuring everything is combined and heated through.
Remove from heat and garnish with chopped parsley. Serve hot with grated Parmesan cheese on the side.