Prep the Beef: Place the beef slices between two sheets of plastic wrap. Gently pound with a meat mallet until they are about 1/4-inch thick. Season both sides of each piece of beef with garlic powder, salt, and black pepper.
Sauté the Mushroom Filling: In a large skillet over medium-high heat, melt 1 tablespoon of butter. Add half of the sliced mushrooms and the minced garlic. Sauté for 4 to 5 minutes until soft and browned. Remove from the heat.
Assemble the Roll Ups: Lay the seasoned beef slices flat. Place one half-slice of Swiss cheese and a spoonful of the sautéed mushrooms onto each piece of beef. Roll the beef tightly around the filling. Secure each roll with one or two wooden toothpicks.
Sear the Beef: Heat the olive oil in the same skillet over medium-high heat. Add the beef rollups seam-side down. Sear for 2 to 3 minutes per side until they develop a deep brown crust. Remove the rollups from the pan and set them aside on a plate.
Make the Gravy: Melt the remaining 1 tablespoon of butter in the skillet. Add the rest of the sliced mushrooms and cook for 3 to 4 minutes. Pour in the beef broth and Worcestershire sauce, scraping up any browned bits from the bottom of the pan. Bring to a simmer.
Simmer and Thicken: Return the beef rollups to the skillet. Cover and simmer on medium-low heat for 15 minutes, or until the beef is tender. Stir in the cornstarch slurry and let the sauce bubble for 1 to 2 minutes until it thickens into a glossy gravy.
Serve: Carefully remove the toothpicks from the beef rollups before serving. Spoon the warm mushroom gravy over the top, garnish with fresh parsley, and serve warm.