Combine the seasoning and whisk well. Start the pasta. Bring a large pot of water to a boil, salt it and cook al dente according to the directions on the package. Reserve 1 cup of pasta water before draining.
While the pasta is cooking, heat olive oil in a large heavy bottomed skillet add the shrimp, sprinkle with a few pinches of the cajun seasoning.
Cook shrimp for 2-3 minutes over medium heat, flipping them halfway (they will cook more when they are added back into the sauce. Take the shrimp out of the pan and set it aside.
Slice the chicken sausage. Add the smoked sausage to the same pan, sprinkle with a few pinches of cajun seasoning and cook for 2-3 minutes until slightly crispy. Take the chicken sausage out of the pan and set aside.
Sauté the veggies. Add chopped shallot and chopped red bell pepper to the same pan, spritz with oil only if needed, sauté for 1-2 minutes, stirring occasionally. Add the garlic and continue to cook for a minute. Add the leftover cajun seasoning and tomato paste.
Next, add in the broth to deglaze the pan and soak up the bits of browning in the pan. In a small bowl, add cornstarch and 1 tsp water, whisk or mix and add it to the cajun broth.
Bring the broth mix to a simmer. Let it simmer for about 3 minutes. Add in the sour cream (if using), shrimp, sausage and cooked pasta. Let it come back to a simmer. If you need to thin the sauce to your liking, use the reserved pasta water otherwise, leave it as is!