Step 1: Bring a large pot of generously salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Reserve about 1 cup of the starchy pasta water before draining.
Step 2: About 3-4 minutes before the pasta is fully cooked, add the cut asparagus pieces to the boiling pasta water. Cook until the asparagus is tender-crisp and bright green.
Step 3: While the pasta and asparagus cook, melt the butter in a large skillet or Dutch oven over medium heat. Add the minced garlic and sauté for 1 minute until fragrant, being careful not to brown it.
Step 4: Add the lemon zest and fresh lemon juice to the skillet with the garlic butter. Stir gently to combine.
Step 5: Drain the pasta and asparagus together. Transfer them directly to the skillet with the lemon-garlic sauce.
Step 6: Add the grated Parmesan cheese and 2-3 tablespoons of the reserved pasta water to the skillet. Toss everything together vigorously, adding more pasta water a tablespoon at a time, until the pasta is beautifully coated in a light, glossy sauce. The starch from the pasta water helps emulsify the sauce.
Step 7: Season the lemon asparagus pasta with salt, freshly ground black pepper, and optional red pepper flakes to taste. Adjust seasonings as needed.
Step 8: Serve immediately, garnished with extra grated Parmesan cheese and fresh chopped parsley. A fresh lemon wedge on the side is perfect for an additional squeeze of citrus.