In a skillet over low heat, melt 1 tablespoon of butter. Add the thinly sliced onions and mushrooms, then splash in Worcestershire sauce. Cover and cook for about 10 minutes, stirring occasionally until they soften and start to caramelize. Add minced garlic and continue to sauté for another 5 minutes.

While the onions and mushrooms are cooking, take your ground beef and gently form it into four equal patties. Avoid overworking the meat. Season both sides with salt and black pepper. Drizzle Worcestershire sauce atop each patty and spread mustard around.

Preheat your grill to medium-high heat. Place the seasoned patties on the grill and cook undisturbed for about 5 minutes. Flip the patties and cook for an additional 3 minutes. Add sharp cheddar cheese atop and cover to melt.

Butter the cut sides of the hamburger buns. In a separate skillet, toast the buns butter-side down until golden.

Assemble by spreading mayonnaise on the bottom half of each bun, adding cheeseburger patties, and topping with sautéed onions and mushrooms. Cover with the top half of the bun.