Start by bringing a large pot of salted water to a rolling boil. Add spaghetti and cook until al dente, about 8-10 minutes. Reserve 1 cup of pasta water, drain, and set aside.

In a skillet, melt butter over medium heat. Add minced garlic and red pepper flakes; sauté for 30 seconds to 1 minute until fragrant.

Pour in lemon juice and reserved pasta water, stir, and bring to a gentle bubble.

Add spaghetti and baby spinach to the skillet, tossing gently until spinach wilts and pasta absorbs flavors.

Remove from heat and stir in lemon zest, Parmesan cheese, and season with salt and black pepper. Serve immediately.