Mix seasoning. Mix the salt, paprika, garlic powder, chili powder, cayenne pepper, and red pepper flakes in a small bowl. Set aside.
Cook the pasta. Bring a large pot of salted water to a boil. Add the fettuccine pasta and cook according to package instructions until al dente. Drain and set aside, tossing with a little oil to prevent sticking.
Cook veggies. Heat a large cast iron skillet or other heavy-duty pan over medium heat. Add the vegetable oil. Once hot, add the chopped onion and bell peppers. Sprinkle with 1 tablespoon of the prepared Cajun seasoning. Cook for 5-7 minutes, stirring occasionally, until the vegetables are slightly softened. Transfer the vegetables to a clean plate and set aside.
Cook meats. In the same skillet, increase heat to medium-high. Add the sliced andouille sausage and cook for 3-4 minutes until browned and slightly crispy on the edges. Stir occasionally to make sure an even browning. Then, add the shrimp and another tablespoon of Cajun seasoning. Cook for 2-3 minutes, stirring occasionally, until the shrimp are pink and cooked. Remove the shrimp and sausage to the plate with the veggies.
Make sauce. Reduce heat to low and add the butter to the skillet. Once melted, pour in the heavy cream and stir to combine with the pan drippings. Add the lemon zest and grated Parmesan cheese. Simmer for 1-2 minutes, stirring frequently, until the sauce is slightly thickened. Taste and adjust seasoning with salt and black pepper if necessary.
Add pasta and garnish. Return the cooked sausage, shrimp, and veggies to the skillet with the sauce. Stir to combine and reheat for about 1 minute. Add the drained pasta to the skillet and toss everything together to coat the pasta evenly with the sauce. Serve immediately, garnished with chopped parsley and additional Cajun seasoning if desired.