Preheat Oven: Preheat your oven to 375°F (190°C). Set aside an 8×8-inch baking dish.
Cook the Beef and Mushrooms: In a high-sided pan over high heat, add the ground beef and salt. Crumble the beef with a stiff utensil as it cooks. Once the beef is browned and has released its liquid, add the sliced mushrooms and garlic to the pan. Cook, stirring occasionally, until the mushrooms have shrunk and most of the liquid has evaporated, about 6 to 8 minutes. The only liquid remaining should be the beef grease.
Assemble the Casserole: Transfer the beef-mushroom-garlic mixture to the prepared baking dish. Stir in the crumbled goat cheese and 2/3 of the chopped Swiss cheese until everything is well combined. The goat cheese should melt quickly and coat the other ingredients nicely. Evenly sprinkle the sliced scallions on top, then distribute the remaining 1/3 of the Swiss cheese over the scallions. Smooth the surface of the casserole so it’s flat and even.
Bake and Serve: Place the baking dish in the preheated oven. Bake the casserole uncovered at 375°F (190°C) until it is very hot, bubbling around the edges, and the cheese is completely melted, about 15 minutes. Let the casserole cool at room temperature for 5 minutes before cutting it into 4 equal portions. Serve and enjoy!