Bring a large pot of heavily salted water to a boil. Cook the spaghetti according to the package directions. Reserve 1 cup of the pasta cooking water. Drain the pasta, rinse with hot water, and set aside.

In a large skillet over medium-high heat, heat 1 tablespoon oil. Add the sausage to the pan and cook, breaking up clumps, until cooked through and browned, about 5 minutes. Remove the sausage from the pan and place on a plate lined with paper towels. Add the remaining 1 tablespoon of oil to the pan and add the Brussels sprouts. Cook, stirring, until the sprouts soften and begin to brown, 5-6 minutes. Add the garlic, minced sage, red pepper flakes, cooked turkey sausage, cooked spaghetti, and ½ cup reserved pasta water. Toss to coat. If the dish seems dry, add a little more of the pasta water. Season to taste with salt and pepper.

Serve with Parmesan cheese and garnish with fresh sage leaves.