Cookwith - Double Smash Burger with Umami Onion Jam

Ingredients

• 1.25 lbs (20 oz) 85% lean ground beef (divided into 4 x 5 oz balls)
• 1 tbsp kosher salt
• 1/2 tsp freshly ground black pepper
• 4 slices sharp cheddar cheese
• 2 brioche burger buns
• 2 tbsp unsalted butter, softened
• 1/2 cup yellow onion, thinly sliced
• 1 tsp soy sauce
• 1 tsp balsamic vinegar
• 1 tsp brown sugar
• 1 tbsp mayonnaise
• 1 tbsp ketchup
• 1 tsp Dijon mustard
• 4 leaves crisp romaine lettuce
• 4 slices ripe tomato