01 - Cook the pasta in a large pot of salted boiling water until al dente. Reserve 1/2 cup pasta water, then drain and set aside.
02 - Heat 1 tablespoon olive oil in a large oven-safe skillet over medium heat. Add sausage, breaking it up with a spoon, and cook until browned and cooked through, about 7–8 minutes. Remove sausage to a plate.
03 - Preheat the broiler or set oven to 450°F.
04 - Add remaining 1 tablespoon olive oil and 1 tablespoon butter to the skillet. Add apple slices and onion; season with salt, pepper, and thyme. Sauté for 5–7 minutes until apples are golden and onions are softened.
05 - Stir in garlic and cook for 1 minute until fragrant.
06 - Return sausage to the skillet. Add the cooked pasta, chicken broth, and reserved pasta water as needed to create a silky sauce. Stir in spinach until wilted.
07 - Dot with remaining 1 tablespoon butter and sprinkle Parmesan evenly over the top.
08 - Place the skillet under the broiler for 2–3 minutes, or until the top is bubbly and golden.
09 - Serve hot, with extra Parmesan and red pepper flakes if desired.