Gather and prepare all ingredients. Preheat the oven to 375°F, grease a 9x13 casserole dish with nonstick cooking spray, and set it aside.

Bring a medium-sized pot of water to a boil for the pasta. Once boiling, add the pasta and cook for about 10 minutes, or just until tender. Drain and cool the pasta in a colander, then place it in a large bowl for later.

While the pasta is cooking, in a medium skillet over medium heat, add the oil and sliced sausage. Brown for 5 minutes on one side, then flip and continue to brown for an additional 5 minutes. Remove the sausage from the pot and set it aside.

Add the mushrooms and onions. Continue cooking for 5 minutes, stirring occasionally.

Melt the butter in the pan, then sprinkle in the flour and cook for 1 minute.

Add the milk, salt, pepper, garlic powder, Italian seasoning, and nutmeg. Cook until the liquid has thickened, about 2 minutes.

Add the kale and cream cheese and stir until the kale is wilted and the cheese is melted, for about 2 minutes.

Add the sausage and any drippings back in and fold to combine. Pour this mixture into the large bowl with the pasta and fold until combined.

Transfer the mixture into the 9x13 casserole dish and spread evenly.

Top evenly with the mozzarella. Bake, uncovered, in the oven for 25-30 minutes.