In a large skillet, melt the butter over medium heat. Add the minced onion, green bell pepper, red bell pepper, and garlic. Sauté for about 5–7 minutes, or until the vegetables are softened and fragrant.
Add the andouille sausage rounds to the skillet and cook for an additional 5 minutes, letting them brown slightly.
In a mixing bowl, combine the cooked rice, fire-roasted tomatoes (with their juice), shrimp, Cajun seasoning, salt, and pepper. Stir well to ensure everything is evenly mixed.
Add the sautéed vegetable and sausage mixture to the bowl with the rice and shrimp. Gently fold everything together until combined.
Preheat your oven to 350°F (175°C). Transfer the mixture into a greased 9x13 inch baking dish, spreading it out evenly.
Sprinkle the grated Parmesan cheese evenly over the top, followed by the plain bread crumbs.
Bake in the preheated oven for 25–30 minutes, or until the casserole is bubbly and the top is golden brown.
Remove the casserole from the oven and let it cool for a few minutes before serving. Enjoy your Louisiana Casserole warm!