Cook pasta according to package directions. Reserve 1/2 cup pasta water before draining.
Toss shrimp with 1 tbsp Cajun seasoning. Heat olive oil in a large skillet over medium-high heat and cook shrimp 1-2 minutes per side until pink. Remove 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.
Add drained pasta and toss to coat. Return shrimp to the pan and toss everything together. Add pasta water if the sauce is too thick.
Stir in Parmesan and top with sliced green onions before serving.