Cook the pasta in a large pot of salted water according to package instructions until al dente, about 8-10 minutes. Drain and reserve ½ cup of the pasta water.

In a large skillet, heat olive oil over medium heat. Add the Italian sausage, breaking it up, and cook for 5-7 minutes until browned. Transfer to a plate.

In the same skillet, add halved Brussels sprouts and diced butternut squash. Cook for 10-15 minutes until tender and caramelized, stirring occasionally.

Melt unsalted butter in a small saucepan over low heat, then add minced garlic and sauté for 1-2 minutes until fragrant.

Combine the sausage, sautéed vegetables, and cooked pasta in the skillet. Pour the garlic butter sauce over and toss until combined.

Adjust the consistency with reserved pasta water, season with salt, pepper, and red pepper flakes to taste. Toss well.

Serve topped with grated Parmesan and garnish with parsley.