Cook Pasta: Bring a large pot of salted water to a boil. Cook the penne pasta according to package directions until al dente. Before draining, reserve 1 cup of the starchy pasta water, then drain the pasta and set it aside.
Brown Beef & Veggies: While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Add the ground beef and cook, crumbling, until browned (about 5-7 minutes). Drain excess grease. Add the chopped onion and sliced mushrooms to the skillet and sauté for 5-6 minutes until softened and browned. Stir in the minced garlic and cook for 1 more minute until fragrant.
Make the Sauce: Sprinkle the flour over the beef mixture and stir for 1 minute to cook out the raw taste. Slowly pour in the beef broth and Worcestershire sauce, stirring constantly and scraping up any browned bits from the bottom of the pan. Bring to a simmer and cook for 3-5 minutes to thicken.
Get Cheesy: Reduce heat to low. Add the cubed Velveeta and stir gently until completely melted and the sauce is smooth.
Combine & Finish: Remove the skillet from the heat. Stir in the room-temperature sour cream until fully incorporated. Add the drained penne pasta to the skillet and toss to coat everything in the sauce. If the sauce is too thick, add a splash of the reserved pasta water. Season with salt and pepper to taste.
Serve: Garnish generously with fresh parsley and serve immediately.
