Preheat the oven to 425ºF. Spray a 9x13-inch baking dish with nonstick cooking spray.

Bring a large pot of salted water to a boil. Add the pasta and cook until al dente. Drain.

Meanwhile, heat 1 tablespoon of the oil in a large skillet over medium heat. Add the onion and the pepper. Season with salt and pepper. Cook until very tender, about 10 minutes. Add the garlic and cook for one minute, then sprinkle the flour over the top. Stir and cook for another minute.

Slowly pour the milk into the onion mixture, whisking to combine. Bring to a simmer, then add in the cream cheese and the cajun seasoning. Cook until the cream cheese has softened, then stir in both the cheddar and the gruyere.

Heat the remaining tablespoon of oil in another skillet. Add the kielbasa and cook, turning, until browned on both sides.

In the pot you used to boil the pasta, combine the drained pasta, the kielbasa and the cheese mixture. Stir to combine, then pour into the prepared baking dish.

In a bowl, mix together the breadcrumbs and butter until all of the breadcrumbs are coated. Sprinkle over the top of the pasta. Place into the preheated oven.

Bake until bubbling and the breadcrumbs have browned, 10-12 minutes. Serve warm.