Bring a large pot of salted water to a vigorous boil. Add the spaghetti and cook according to package instructions, usually around 9-10 minutes, until al dente.

In a large skillet, melt 4 tablespoons of unsalted butter over medium heat. Once melted and bubbling, add 3 minced garlic cloves and a pinch of red pepper flakes.

Pour in the juice of one fresh lemon and the reserved pasta water, stir well and let it bubble for about 2 minutes.

Add the drained spaghetti and 4 cups of baby spinach to the skillet, tossing gently to coat. Cook for an additional 2-3 minutes until the spinach wilts.

Remove from heat and mix in the zest of one lemon, 1 cup of grated Parmesan cheese, and a sprinkle of salt and pepper.

Plate the Spinach Lemon Pasta while it's warm, adding extra grated Parmesan and pepper if desired.