Cook the penne pasta according to package instructions until al dente and drain.
In a large skillet, heat a splash of oil over medium heat and add the Cajun sausage, cooking until browned.
Add the diced onion and bell pepper to the skillet, stirring until softened, about 5-7 minutes.
Stir in the minced garlic and cook for an additional minute until fragrant.
Pour in the diced tomatoes and their juice, scraping any bits off the bottom of the pan.
Stir in the heavy cream and Cajun seasoning. Let it simmer for 5 minutes to thicken.
Add the cooked pasta to the skillet, tossing to coat evenly with the sauce.
Season with salt and pepper to taste, and garnish with sliced green onions before serving.