Toss the shrimp with 1 tablespoon of Cajun seasoning and set aside.
In a large, deep skillet or pot, heat the butter and olive oil over medium-high heat. Add the shrimp and cook for 2-3 minutes per side until pink and cooked through. Remove shrimp and set aside.
In the same pot, add the diced bell pepper and cook for 2-3 minutes until softened. Add the garlic and cook for 1 minute more.
Add the fettuccine, chicken broth, heavy cream, diced tomatoes, and remaining 1 tablespoon Cajun seasoning. Stir to combine.
Bring to a boil, then reduce heat to medium and cook for 10-12 minutes, stirring frequently, until pasta is al dente and sauce has thickened.
Stir in the Parmesan cheese until melted, then add the cooked shrimp back to the pot. Toss to coat.
Taste and adjust seasoning with salt if needed. Garnish with sliced green onions and serve immediately.