1. Bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions until al dente. Reserve 1 cup of pasta water, then drain.
2. In a large skillet, heat olive oil over medium heat. Add sliced garlic and sauté for 1–2 minutes, until fragrant and lightly golden. Do not let it burn.
3. Add the spinach and cook for 2–3 minutes, stirring frequently, until wilted. Add red pepper flakes if using.
4. Add the cooked spaghetti to the skillet. Splash in some reserved pasta water to loosen and help coat the noodles. Season with salt and pepper. Toss well to combine.
5. Plate immediately. Top with grated Parmesan cheese and a squeeze of lemon juice for brightness.