Step 1: Bring a large pot of generously salted water to a boil. Add the asparagus pieces and blanch for 2-3 minutes until bright green and tender-crisp. Immediately transfer the asparagus to an ice bath to stop the cooking; drain well and set aside.
Step 2: In the same pot of boiling water, cook the pasta according to package directions until al dente. Before draining, reserve about 1 cup of the starchy pasta water. Drain the pasta and set aside.
Step 3: While the pasta cooks, 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 let it brown.
Step 4: Pour in the chicken or vegetable broth and bring to a gentle simmer. Cook for 2-3 minutes, allowing it to reduce slightly.
Step 5: Stir in the heavy cream, lemon zest, and red pepper flakes (if using). Bring the sauce to a gentle simmer and cook for another 2-3 minutes until it slightly thickens. Season with salt and black pepper to taste.
Step 6: Add the cooked pasta to the skillet with the sauce. Toss to coat evenly. Add the blanched asparagus and half of the grated Parmesan cheese. Toss again.
Step 7: If the sauce seems too thick, gradually add some of the reserved pasta water, a tablespoon at a time, until you reach your desired consistency.
Step 8: Stir in the fresh lemon juice just before serving. Taste and adjust seasonings one last time. Serve immediately, garnished with the remaining Parmesan cheese and fresh chopped parsley.