Start by cooking the pasta according to package instructions. Reserve ½ cup of the pasta water and avoid rinsing, as the starch helps the sauce cling.
Melt butter in a skillet over medium heat. Add garlic and red pepper flakes, sauté until fragrant, then stir in lemon juice and a splash of pasta water for a light, creamy consistency.
Toss the cooked spaghetti with the sauce and fresh spinach. Stir until the spinach wilts and pasta is fully coated in the lemony garlic butter.
Remove from heat, sprinkle in lemon zest and freshly grated parmesan. Season with salt and black pepper to taste, then serve immediately.