Heat the olive oil in a large sauté or saucepan over medium-low heat. Add the garlic and red pepper flakes and cook gently for about 3 minutes, until the garlic is lightly browned. Add the spinach and cook, moving the uncooked spinach to the bottom of the pan with tongs, for about 5 minutes, until fully wilted. Drain off any excess water from the bottom of the pan. Stir in the lemon juice and season to taste with salt and black pepper.