Bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions until al dente.

In the last 3 minutes of cooking, add the asparagus to the pot. Drain the pasta and asparagus together, reserving 1/2 cup of the pasta water.

In a large skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant.

Add the lemon juice, lemon zest, and red pepper flakes (if using) to the skillet, stirring to combine.

Add the drained spaghetti and asparagus to the skillet, tossing to coat in the lemon sauce.

Gradually add reserved pasta water as needed to create a light sauce that coats the pasta.

Stir in the grated parmesan cheese and season with salt and black pepper to taste.

Divide the pasta among serving plates. Garnish with chopped fresh parsley and additional parmesan cheese if desired.

Serve immediately while hot.