Bring a large pot of salted water to a rolling boil over high heat. Add the spaghetti and cook for 9-10 minutes or until al dente, stirring occasionally. Reserve 1 cup of pasta water before draining the spaghetti.

In a large skillet, melt 4 tablespoons of unsalted butter over medium heat. Add the minced garlic and red pepper flakes, sautéing for about 30 seconds until fragrant.

Pour in the juice of one fresh lemon along with the reserved pasta water into the skillet, bringing the mixture to a gentle bubble. Stir well for about 1-2 minutes.

Add the drained spaghetti and baby spinach directly into the skillet. Toss everything together using tongs for 2-3 minutes until the spinach wilts.

Remove the skillet from the heat and stir in the lemon zest, Parmesan cheese, salt, and black pepper. Toss gently to combine.

Transfer the Spinach Lemon Pasta to serving plates or a large bowl. Optionally, garnish with additional lemon zest and Parmesan cheese.