Preparation:

Step 1: Caramelize the Onions

1. Heat butter and olive oil in a skillet over medium heat.
2. Add the sliced onions and cook for 10-15 minutes, stirring occasionally, until the onions become soft and golden brown.
3. Stir in the brown sugar and balsamic vinegar (if using) to enhance the sweetness. Cook for an additional 5 minutes.
4. Remove from heat and set aside.

Step 2: Prepare the Burger Patties

1. Divide the ground beef into 6 loose balls, about ¼ pound each.
2. Heat a griddle or cast-iron skillet over medium-high heat until hot.
3. Place a ball of ground beef on the griddle and press it down firmly with a spatula to create a thin patty. Season with salt and pepper.
4. Cook for 2-3 minutes without moving to allow a crispy crust to form.
5. Flip the patty and cook for another minute. Place a slice of cheddar cheese on top and let it melt.
6. Repeat with the remaining beef balls.

Step 3: Toast the Buns

1. Spread a thin layer of butter on the cut sides of the burger buns.
2. Toast the buns on a skillet or griddle until golden and slightly crispy.

Step 4: Assemble the Cheeseburger

1. Spread a layer of BBQ sauce on the bottom half of the bun.
2. Place a cheesy beef patty on top, followed by a generous portion of caramelized onions.
3. Add a second and third patty, layering more onions and BBQ sauce between each.
4. Top with optional toppings such as lettuce, pickles, or mayonnaise.
5. Finish with the top bun to complete the stack.