Minecraft MooShroom Smash Burgers

Ingredients

• 1 1/3 lb ground beef
• romaine lettuce
• Swiss cheese
• 4 deluxe hamburger buns
• 1/4 cup mayonnaise
• 2 tsp minced garlic
• 2 tsp lemon juice
• 1/4 tsp kosher salt
• 1/4 tsp black pepper
• 4 tbsp butter
• 8 oz assorted mushrooms, broken apart by hand into chunks
• 1 small onion, halved and sliced
• 2 tbsp balsamic vinegar
• salt and pepper