In a large pot, bring salted water to a boil. Add 12 ounces of fettuccine and cook according to the package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta and set aside.
In a large skillet, melt 4 tablespoons of butter over medium heat. Add the sliced andouille sausage and cook for about 3 minutes until browned. Then, add the shrimp, Cajun seasoning, and paprika. Cook until the shrimp are pink and opaque, about 3–4 minutes. Remove the shrimp and sausage from the skillet and set aside.
In the same skillet, add the remaining 4 tablespoons of butter. Once melted, add the sliced green onions, diced yellow and green peppers, and sliced mushrooms. Sauté for about 5–7 minutes until the vegetables are tender.
Add the minced garlic and diced tomato to the skillet with the vegetables, cooking for an additional 2 minutes until fragrant.
Pour in 1 cup of heavy cream and stir to combine. Allow the mixture to simmer for 3–4 minutes until slightly thickened. Stir in the lemon juice and season with salt and pepper to taste.
Add the cooked fettuccine, shrimp, and sausage back into the skillet. Toss everything together, adding a bit of reserved pasta water if needed to loosen the sauce. Mix until the pasta is well coated.
Remove the skillet from heat and stir in the freshly grated Parmesan and chopped parsley. Adjust seasoning if necessary.
Serve the Mardi Gras Pasta immediately, garnished with extra parsley and Parmesan if desired. Enjoy this festive dish that brings a taste of New Orleans to your table!