Bring a large pot of generously salted water to a rolling boil. Add the penne and cook until 1 minute shy of al dente. Before draining, scoop out 1/4 cup of the pasta cooking water and set aside. Drain the pasta and set it aside.

In a large skillet, melt 2 tablespoons of the butter with the olive oil over medium heat. Add the diced onion and cook, stirring occasionally, until translucent, about 1 to 2 minutes.

Add the carrots and cook for 1 minute, then stir in the broccoli and cook 1 minute more. Add the bell pepper and asparagus, cooking for another 1 to 2 minutes, then add the minced garlic. If the pan looks dry, especially before adding the asparagus, add the remaining 1 tablespoon of butter. Season the vegetables lightly with salt and pepper, then transfer them to a plate and set aside.

Return the empty skillet to the stove. Add the remaining 1 tablespoon butter, chicken broth, lemon juice, and the reserved pasta water. Bring to a boil, scraping up any browned bits from the bottom of the pan, and let it simmer for 2 to 3 minutes.

Add the cooked pasta to the skillet and continue to simmer for 3 to 4 minutes, stirring occasionally, until the sauce reduces by about half and lightly coats the noodles.

Return the cooked vegetables to the skillet and toss everything together until well coated. Taste and adjust the seasoning with additional salt and pepper if needed. Serve immediately, topped with freshly grated Parmesan and minced parsley. Add lemon wedges on the side if desired.