INGREDIENTS  

• 1 lb ground beef
• 1 large onion, sliced
• 8 oz sliced mushrooms, divided
• 2 tbsp Worcestershire sauce
• 1 tsp kosher salt
• 1 tsp black pepper
• 6 slices provolone cheese
• 6 slices rye bread
• 6 tbsp butter

INSTRUCTIONS 

• Melt 3 tablespoons butter in a large skillet or griddle over medium heat. Add sliced onion and cook until onions are golden brown, caramelized and soft, about 15-20 minutes. Stir them often to prevent them from burning, adding more butter if needed.
• Once onions are almpst done, add half the sliced mushrooms to the skilet and cook alongside the onions until they have released all their moisture and begin to brown. Remove grilled onions and mushrooms from the pan and set aside; keep warm.
• While onions and mushrooms are cooking, chop the remaining half of the mushrooms into a rough chop.
• Add mushrooms to a large mixing bowl and combine with ground beef, Worcestershire sauce, salt and pepper.
• Form beef mixture into 3 equal sized patties, pressing them out slightly larger than the shape of the bread slices.
• Grill patties over medium high heat, pressing the patties flat with the back of a spatula to prevent them from shrinking too much. Cook patties to a medium well doneness.
• While patties are cooking, spread butter onto one side of each slice of bread.
• Once patties are cooked to your desired doneness, remove them from the skillet. Place ½ the bread slices buttered side down in the skillet.
• Top each slice of bread with a slice of Provolone cheese. Then top with a grilled beef patty, then a mound of grilled onions and mushrooms.
• Add another slice of Provolone cheese and place a slice of bread on top, buttered side facing up.
• Once first slice of bread is toasted and golden brown, use a spatula to carefully flip the entire sandwich over to grill the other slice of bread.
• Once both slices of bread are grilled to a crunchy golden brown and cheese is hot and melted, transfer each sandwich to a cutting board.
• Cut each sandwich in half with a sharp knife to serve.