Bring a large pot of salted water to a boil. Add and cook the spaghetti according to package instructions (typically 9 to 10 minutes). Reserve 1 cup of pasta water before draining the spaghetti, do not rinse.
In a large skillet, over medium heat, melt the unsalted butter. Add the minced garlic and red pepper flakes, stir and cook until fragrant (about 30 seconds to 1 minute).
Add the lemon juice and ½ cup of pasta water. Once it begins to bubble, add the spinach and pasta. Toss together until spinach wilts and the sauce coats the noodles. NOTE: If the pasta sauce seems dry, add a ¼ cup of pasta water.
Remove skillet from heat, add lemon zest, parmesan cheese, salt, and black pepper. Toss a few times before serving.
Notes
For a creamier sauce, add a tablespoon of heavy cream or Greek yogurt. Adjust the amount of red pepper flakes according to your desired level of spiciness.