Begin by boiling a large pot of salted water over high heat. Once boiling, carefully add the spaghetti and cook according to package instructions, typically 9-10 minutes, until al dente. Reserve 1 cup of the pasta cooking water before draining the spaghetti; do not rinse it.

In a large skillet, melt 4 tablespoons of unsalted butter over medium heat. Once melted and bubbling, add 3 minced garlic cloves and ½ teaspoon of red pepper flakes. Sauté for about 30 seconds or until fragrant.

Pour in the juice of one lemon and ½ cup of reserved pasta water into the skillet. Let it simmer until bubbling, then add drained spaghetti and 4 cups of fresh baby spinach. Toss together until spinach wilts.

Remove from heat and stir in the zest of one lemon, ½ cup of freshly grated Parmesan cheese, ½ teaspoon of salt, and a few cracks of black pepper. Toss until well combined.