Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package instructions until al dente, usually about 8-10 minutes. Before draining, reserve 1 cup of the pasta cooking water in a bowl for later use. Drain the spaghetti, but do not rinse it.

In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add 3 minced garlic cloves and a pinch of red pepper flakes. Sauté for 30 seconds to 1 minute, until the garlic is fragrant and lightly golden.

Pour in the juice of one lemon and half a cup of the reserved pasta water into the skillet. Stir gently and bring the mixture to a gentle simmer.

Carefully add the drained spaghetti and a generous handful of fresh baby spinach to the skillet. Toss everything together using tongs until the spinach wilts and the pasta is evenly coated in sauce.

Remove the skillet from heat and add the zest of one lemon, ¼ cup of freshly grated Parmesan cheese, a pinch of salt, and freshly cracked black pepper. Toss to ensure well incorporation.