Bring a large pot of salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Drain and set aside, reserving a cup of pasta water.

In a large skillet over medium heat, heat olive oil. Add the diced onion and minced garlic, sautéing until fragrant, about 2-3 minutes.

Add shrimp to the skillet and cook until they turn pink, about 3-4 minutes. Gently fold in the lump crab meat.

Pour in the heavy cream and Cajun seasoning, stirring well. Let the mixture simmer for about 5 minutes. Adjust salt and pepper to taste.

Once the sauce is thickened, add the cooked pasta directly to the skillet. If too thick, gradually add reserved pasta water until desired consistency is reached.

Plate the creamy Cajun pasta, garnish with fresh parsley, and serve warm.