In a small skillet, add 1 tbsp butter, mushrooms, onions, worcestershire sauce, and red wine vinegar over low heat. Cover and cook for about 10 minutes, stirring occasionally. Add minced garlic, cover and cook for an additional 5 minutes. Remove lid, stir, and let any excess liquids cook off.
While the onions and mushrooms are cooking, form four hamburger patties. Try not to overwork the meat for best results. Salt and pepper both sides of all four patties. Drizzle about a 1/2 tsp of worcestershire on each patty. Then, rub with 1/2 tsp mustard on all sides.
Heat your grill or grill pan to medium-high heat. (You can also use a cast iron skillet, or just a regular skillet) Add the hamburger patties to the hot grill and cook for 5 minutes, undisturbed. Flip patties over and cook for an additional 3 minutes. Place a cheese slice on each patty and turn off heat. Cover and let cheese melt.
Butter the hamburger buns and heat a skillet to medium-high heat. Lay buns butter side down in skillet and cook until golden.
Top each bun with mayonnaise, mustard, a cheeseburger patty, grilled onions and mushrooms.
You are ready to serve! Enjoy!