In a large pot, bring salted water to a boil. Add the penne pasta and cook according to package instructions until al dente. Drain and set aside, reserving about 1 cup of the pasta water.
In a large skillet, heat olive oil over medium heat. Add diced onions and sauté for 2-3 minutes until they become translucent. Stir in minced garlic and cook for another 30 seconds until fragrant.
Add the sliced cremini mushrooms to the skillet. Cook for 5-7 minutes until they are golden brown and have released their moisture.
Toss in the asparagus pieces and sauté for another 3-4 minutes until they become tender yet still vibrant.
Pour in the heavy cream, stirring until combined. Add the grated Parmesan cheese, thyme, lemon zest, salt, and pepper. Stir continuously until the cheese has melted and the sauce is creamy. If the sauce is too thick, add reserved pasta water slowly until the desired consistency is achieved.
Add the cooked penne to the skillet, tossing it in the creamy mushroom-asparagus sauce until well coated. Adjust seasoning as needed.
After the pasta is evenly coated, remove from heat. Plate the creamy mushroom asparagus penne and garnish with chopped fresh parsley.
Notes
Adjust seasoning to taste and add more pasta water for desired sauce consistency.