Cook pasta according to package directions. Once the pasta is ready, reserve ½ cup of pasta water before draining. Set pasta aside.
Cook the sausage:
Slice the sausage horizontally into ½-inch slices or remove it from the casing (your choice) and add it back into the pasta pot or a large skillet.
Cook the sausage if it's uncooked or warm it up if it's pre-cooked, over medium-high heat, stirring, for about 3 minutes or until the sausage is no longer pink. Remove from the skillet or pot with a slotted spoon onto a plate but leave the drippings.
Make the sauce:
Add the butter and melt it over medium-high heat. Add the bell peppers and onion, and stir, cooking them down until they've softened for about 3 to 5 minutes.
Add the seasoning and heavy cream or half and half, and stir to combine with the veggies. Reduce the heat to medium-low and let it simmer to thicken up for about 5 minutes.
Combine:
Add the Parmesan cheese to the sauce and stir to combine. Then the cooked sausage and the cooked pasta. Stir to combine everything. The sauce will be very thick. Now, slowly add the reserved pasta water until you achieve the sauce consistency you like. I usually start with ¼ cup and then slowly add the rest. Turn off the heat.
Serve the Cajun Alfredo pasta topped with additional Parmesan cheese.
Notes
If you want to make the sauce thicker, add about ¼ cup of Parmesan cheese to the pasta mixture. Combine and wait a couple of minutes for it to absorb. Alternatively, you can make a bit of slurry by mixing 1 teaspoon of cornstarch and 2 tablespoons of water. Once mixed, add it to the skillet and simmer for a few minutes to allow it to thicken into the sauce. You should not need to thicken this sauce if you follow the recipe.
I've made this numerous times with gluten-free pasta. Delicious! For a grain-free option, omit pasta, and use 6 peeled, diced, and cooked zucchinis.