1. Bring a large pot of salted water to a boil. Cook the spaghetti until al dente. Reserve ½ cup of the pasta water, then drain the pasta.

2. In a large skillet over medium heat, cook the Italian sausage, breaking it into crumbles, until browned and fully cooked. Remove and set aside.

3. Reduce heat to low. In the same skillet, add butter. Once melted, add minced garlic and sauté until fragrant, about 1 minute.

4. Stir in baby spinach and cook until wilted, about 2–3 minutes. Return sausage to the pan and mix well.

5. Add the cooked spaghetti and toss with the garlic butter mixture. Gradually stir in reserved pasta water until everything is evenly coated. Season with salt, pepper, and red pepper flakes.

6. Transfer mixture to an oven-safe dish. Top with mozzarella and Parmesan cheese.

7. Broil for 2–3 minutes until the cheese is golden and bubbling. Serve hot.