Preheat the oven to 350 degrees F (175 degrees C).
Crumble the ground beef into a large skillet over medium-high heat. When it starts to release some juices, add the onion. Season with Worcestershire sauce and pepper. When the beef is nearly completely browned, add the mushrooms, and saute them for a couple of minutes. Drain off any excess grease, and stir in the cream of mushroom soup and spinach.
Transfer the beef mixture to a 9x13 inch baking dish. Top with slices of Swiss cheese. Neatly layer the potato rounds over the top.
Bake for 45 to 60 minutes in the preheated oven, until the potato rounds are toasted.