Bring a large pot of salted water to a rolling boil. Add spaghetti and cook until al dente, about 9-10 minutes. Reserve 1 cup of pasta water, drain spaghetti.

Melt butter in a skillet over medium heat. Add minced garlic and red pepper flakes, cooking for 30 seconds to 1 minute until fragrant.

Squeeze in lemon juice and stir in reserved pasta water. Bring to a simmer and add baby spinach, stirring until wilted, about 2 minutes.

Add drained spaghetti to the skillet and toss vigorously until well-coated. Add more pasta water if sauce is too thick, about 1-2 minutes.

Remove from heat and stir in lemon zest and grated Parmesan cheese. Season with salt and black pepper to taste; serve immediately.