Cook pasta: Boil penne in well-salted water until al dente. Reserve 1/2 cup pasta water; drain.
Sear chicken: Season chicken with 1/4 tsp salt and 1/4 tsp pepper. Heat olive oil in a large skillet over medium-high. Sauté chicken 4–6 minutes until just cooked. Transfer to a plate.
Sauté mushrooms: Add butter to the skillet. Cook mushrooms with a pinch of salt 4–5 minutes until browned and moisture evaporates.
Add asparagus & garlic: Stir in asparagus and cook 2–3 minutes until bright green and crisp-tender. Add garlic and red pepper flakes; cook 30–60 seconds until fragrant.
Build the sauce: Pour in chicken broth; simmer 2 minutes, scraping browned bits. Reduce heat to medium-low, add cream, and simmer gently 2–3 minutes until slightly thickened.
Finish: Return chicken to the pan. Stir in Parmesan, lemon zest, and lemon juice. Season with remaining pepper and salt to taste. If too thick, add reserved pasta water a splash at a time.
Toss pasta: Add penne to the skillet and coat in sauce. Warm 1–2 minutes.
Serve: Plate creamy mushroom asparagus chicken penne and garnish with parsley and extra Parmesan.