Cook the pasta in a large pot of well-salted boiling water until al dente, according to package directions. Before draining, reserve about 1 cup of the pasta cooking water.

While the pasta cooks, heat the olive oil in a large skillet over medium heat. Add the sliced garlic and cook for 1 to 2 minutes, until fragrant and just beginning to turn golden. Do not let it burn.

Add all the fresh spinach to the skillet. Cook, stirring often, until the spinach is wilted, about 2 to 3 minutes. Stir in the red pepper flakes, if using.

Add the drained spaghetti to the skillet with the spinach and garlic. Pour in about 1/4 cup of the reserved pasta water. Toss everything together until the pasta is well coated and glossy, adding more pasta water if needed. Season generously with salt and black pepper to taste.

Serve immediately, topped with plenty of freshly grated Parmesan cheese and a squeeze of lemon juice from the wedges.