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Melt butter in a pan over low heat. Slice whole mushrooms, and stir them gently into the melted butter. Cover the pan with a lid and let the mushrooms tenderize and cook down. Continue cooking the mushrooms, adjusting the heat as necessary to avoid burning, until the edges start caramelizing in the butter. Stir red wine vinegar, salt, and onion powder into the pan. Cover the pan and continue cooking until the liquid bubbles and reduces until you’re left with a caramelized mushroom topping.
Set the mushrooms aside and turn on an oven to preheat to 350 degrees. Spray a glass 9×13 baking dish with pan spray. In a large mixing bowl, combine ground beef, egg, parmesan cheese, seasoned salt, pepper, mustard powder, garlic powder, onion powder, and thyme to make the casserole filling. Press the beef filling into the prepared baking dish and press it into all the corners and edges. Poke several holes all the way through the beef filling to help avoid shrinkage in the oven later.
Pour the cooked mushrooms and any sauce over the beef filling, spreading it out evenly. Add one slice of tomato per serving on top of the casserole, followed by an even layer of shredded lettuce. Arrange a final layer of swiss cheese slices on the top of the casserole. You can cut and arrange pieces as pictured to make sure the layer is even. Bake the casserole for 30 minutes or until the cheese is melted and golden and the beef passes a meat thermometer check. After cooling the casserole for 5 minutes, slice and serve.