Bring a large pot of salted water to a boil. Add pasta and cook until just before al dente, about 7–8 minutes. Reserve 1 to 1½ cups of pasta water before draining. Set pasta aside.

In a large skillet, melt butter over medium heat. Add diced onion and sauté until soft and translucent, 4–5 minutes.

Stir in garlic and cook until fragrant, about 1 minute.

Sprinkle in flour and stir constantly for 1–2 minutes, until raw flour smell is gone.

Slowly whisk in milk, making sure there are no lumps. Add nutmeg and cayenne. Season with salt and pepper.

Bring to a gentle boil and cook, stirring often, for 1 minute until slightly thickened.

Stir in the spinach and mix until evenly combined.

Fold in cooked pasta. Add reserved pasta water a bit at a time if needed to loosen the sauce.

Taste and adjust salt and pepper as needed. Serve warm and enjoy!