Bring a large pot of salted water to a rolling boil and cook the pasta until just al dente. You want it tender but with a slight bite—it’ll finish cooking in the oven. Drain and set aside, giving it a quick toss with olive oil so it doesn’t stick.

Heat olive oil in a large skillet over medium heat. Add the sliced kielbasa and cook until lightly browned and caramelized on the edges. That smoky aroma filling your kitchen? That’s flavor building already.

Add the chopped onion to the skillet and sauté until soft and translucent. Stir in the garlic and cook just until fragrant—about 30 seconds. This is where the casserole really starts to smell irresistible.

Lower the heat and pour in the heavy cream and milk. Stir gently, then add paprika, black pepper, salt, and red pepper flakes. Let it simmer for a few minutes until slightly thickened, silky, and warm.

Stir in the cheddar and mozzarella, a handful at a time, until melted and smooth. Finish with Parmesan for depth. The sauce should be creamy, cheesy, and spoon-licking good.

Add the cooked pasta and optional veggies to the skillet, tossing gently until everything is coated in that luscious sauce. Transfer the mixture to a greased baking dish and smooth the top.

Bake at 375°F (190°C) for 25–30 minutes, or until the casserole is bubbling around the edges and lightly golden on top. Let it rest for 5 minutes before serving.