Preheat your oven to 375°F (190°C) to prepare for baking.
In a large pot of salted boiling water, cook the penne pasta according to package instructions until it is al dente. Once cooked, drain the pasta and set it aside.
Heat the olive oil in a large skillet over medium heat. Add the diced onion and bell pepper, sautéing them until they soften, about 5 minutes.
Add the minced garlic to the skillet and continue to cook for an additional minute, stirring frequently until the garlic becomes fragrant.
Season the peeled and deveined shrimp with Cajun seasoning, salt, and black pepper. Add the seasoned shrimp to the skillet and cook until they turn pink and opaque, approximately 3-4 minutes.
Pour in the heavy cream and the diced tomatoes, bringing the mixture to a gentle simmer for 2-3 minutes to allow the flavors to meld.
Carefully fold the cooked penne pasta and half of the shredded cheddar cheese into the shrimp mixture in the skillet, ensuring everything is well combined.
Transfer the pasta mixture to a greased baking dish, spreading it out evenly.
Sprinkle the remaining shredded cheddar cheese over the top, followed by the grated Parmesan cheese for an extra layer of flavor.
Place the baking dish in the preheated oven and bake for 20-25 minutes, or until the top is golden brown and bubbling.
Once done, carefully remove the baking dish from the oven and let it cool for a few minutes. Before serving, garnish with freshly chopped parsley for a pop of color and freshness.