Bring a large pot of salted water to a rolling boil. Add the spaghetti and cook according to package instructions until al dente. Reserve 1 cup of pasta water, then drain.

In a large skillet, melt the butter over medium heat. Add minced garlic and red pepper flakes, sauté for 30 seconds until fragrant.

Pour in lemon juice and half a cup of reserved pasta water, stirring gently. Bring to a simmer and add spinach, cooking until wilted (about 2-3 minutes).

Add drained spaghetti to the skillet, tossing to combine with the sauce and spinach. Adjust sauce consistency with reserved pasta water if necessary.

Remove from heat and stir in lemon zest, grated Parmesan, salt, and black pepper. Mix thoroughly.

Serve immediately, garnished with additional Parmesan and a wedge of lemon if desired.