Boil pasta water: Bring a large pot of salted water to boil for the pasta.
Prep ingredients: Cut chicken into bite-sized pieces and season with salt and pepper. Trim asparagus and cut into 2-inch pieces.
Cook pasta: Add pasta to boiling water and cook according to package directions.
Cook chicken: While pasta cooks, heat olive oil in a large skillet and cook chicken until golden and no longer pink.
Add garlic: Stir minced garlic into the cooked chicken and cook for 30 seconds until fragrant.
Cook asparagus: Add asparagus to the pasta water during the last 2 minutes of cooking time.
Reserve water: Before draining, scoop out 1/4 cup of pasta water and set aside.
Combine everything: Drain pasta and asparagus, then add to the skillet with chicken and garlic.
Create sauce: Add lemon juice, zest, butter, and a splash of reserved pasta water. Toss everything together.
Finish dish: Stir in Parmesan cheese and red pepper flakes (if using), then sprinkle with fresh chopped parsley. Season to taste with additional salt and pepper if needed.