Cook spaghetti according to package directions until al dente. Reserve about 1 cup of pasta water before draining.
While pasta is cooking, heat olive oil in a large skillet over medium heat. Add minced garlic and red pepper flakes and cook until fragrant, about 1-2 minutes, being careful not to burn the garlic.
Stir in the fresh lemon juice and about 0.5 cup of the reserved pasta water. Bring to a simmer.
Add the drained spaghetti to the skillet. Toss to coat the pasta with the lemon garlic sauce.
Stir in the grated Parmesan cheese and chopped fresh parsley. Add more pasta water, a tablespoon at a time, if needed to reach desired sauce consistency.
Season with salt and black pepper to taste. Serve immediately.
Notes
For an extra touch of flavor, you can add a little lemon zest along with the lemon juice.