1. Bring a large pot of salted water to a boil. Cook the spaghetti until al dente, according to package directions. Reserve 1/2 cup of pasta water, then drain.

2. In a large skillet over medium heat, cook the Italian sausage (removed from casing), breaking it up with a spoon until fully browned and crispy around the edges. Transfer the sausage to a plate.

3. In the same skillet, reduce heat to low and add the butter. Once melted, add the minced garlic and cook for about a minute until fragrant but not browned.

4. Pour in the heavy cream and stir to combine. Let it simmer gently for 2 to 3 minutes. Stir in the grated parmesan and a pinch of red pepper flakes if using.

5. Add the cooked spaghetti to the sauce, tossing gently to coat. Use reserved pasta water as needed to loosen the sauce. Add the spinach and cooked sausage. Toss again until the spinach wilts and everything is well coated.

6. Season with salt and black pepper to taste. Serve hot, with extra parmesan on top if desired.