Bring a large pot of salted water to a rapid boil over high heat. Add the spaghetti and cook according to the package instructions until al dente, about 8-10 minutes. Reserve 1 cup of pasta water, then drain the spaghetti and set aside.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add thinly sliced garlic and sauté for 1-2 minutes until golden and fragrant.

Add 4 cups of fresh spinach to the skillet and cook for 2-3 minutes until wilted and bright green. Sprinkle red pepper flakes if desired.

Add the drained spaghetti to the skillet with the wilted spinach. Pour in the reserved pasta water gradually, about ¼ cup at a time, and season with salt and pepper. Toss thoroughly.

Plate the spaghetti and sprinkle with freshly grated Parmesan cheese and squeeze lemon over each plate before serving.