In a large skillet (12") with a lid or a Dutch oven, melt the butter over medium heat. Sauté the shrimp until uniformly pink. Remove with a slotted spoon and set aside.
Add the sausage and sauté until beginning to brown. Add the white part of the scallions and the diced bell pepper. Cook until the vegetables are tender, stirring frequently. Add the Creole, Cajun, or blackening seasoning and stir around for a minute more.
Pour in the stock, Alfredo sauce, and water. Add the pasta and stir well. Bring to a simmer and turn the heat down a little. The goal is to maintain a gentle simmer. Cover the skillet and allow to cook, stirring occasionally, until the pasta is nearly al dente. You'll need to test it. When the pasta is close to al dente, uncover it and continue to cook until the pasta is as tender as you like.
When the pasta is ready, turn off the heat. Stir in the shrimp, and the sherry if using. Taste for seasoning and adjust. Sauce will thicken a bit upon standing
Garnish with the green sliced scallions and parmesan.