Cook the fettuccine according to package instructions until al dente, then drain.
In a large skillet, heat olive oil over medium heat and sauté the onions until translucent, about 3-4 minutes.
Add the minced garlic to the skillet and sauté for an additional 1-2 minutes, until fragrant.
Add the shrimp to the skillet, cooking until pink and opaque, about 3-4 minutes.
Stir in the lump crab meat gently to avoid breaking it apart.
Pour in the heavy cream, chicken broth, and Cajun seasoning, stirring to combine.
Allow the sauce to simmer for about 5-7 minutes, letting it thicken slightly.
Add the cooked fettuccine to the skillet and toss to coat in the sauce, seasoning with salt and pepper to taste.
Serve the pasta garnished with chopped parsley for a fresh finish.