Cook pasta according to package directions. Reserve 1/2 cup pasta water before draining.
Toss shrimp with 1 tbsp Cajun seasoning. Heat oil in a large skillet over medium-high heat; cook shrimp 1-2 minutes per side until pink. Transfer to a plate.
In the same skillet, melt butter over medium heat. Add bell pepper and cook 3 minutes. Add garlic and remaining Cajun seasoning and cook 1 minute.
Add tomatoes and cook 2 minutes. Pour in chicken broth and heavy cream. Bring to a simmer and cook 4-5 minutes until slightly thickened.
Toss pasta with sauce; return shrimp and toss. Thin with pasta water if needed.
Stir in Parmesan and top with sliced green onions before serving.