Start by bringing a large pot of salted water to a rolling boil. Add the penne pasta and cook it according to package instructions until it's al dente, usually about 9-11 minutes. Drain in a colander and set aside.

In a large skillet over medium heat, melt 2 tablespoons of unsalted butter until bubbly. Add the diced onion and sauté for 3-4 minutes until soft and translucent.

Increase the heat slightly and add the ground beef to the skillet, breaking it up with a spatula. Cook for about 5-7 minutes until browned and no longer pink. Drain any excess fat.

Stir in the sliced mushrooms and minced garlic, cooking for an additional 3-4 minutes until the mushrooms are tender and slightly golden.

Pour in 2 cups of low-sodium beef broth, using a spatula to scrape up any browned bits. Allow this mixture to simmer for 3-5 minutes.

Reduce the heat to low and add the cubed Velveeta cheese, stirring gently until it melts completely, about 2-3 minutes.

Remove from heat and whisk in 1 cup of sour cream until silky and well combined. Adjust seasoning with salt and pepper.

Add the drained penne pasta to the creamy beef sauce and gently toss to coat evenly.

Sprinkle chopped fresh parsley over the top for garnish. Serve hot.