Ingredients

• 1 pound ground beef
• 3 yellow onions, thinly sliced
• 4 sharp cheddar cheese
• 8 bread
• 2 tablespoon butter
• 1 tablespoon olive oil
• 2 tablespoon balsamic vinegar
• 1 tablespoon Worcestershire sauce
• 1 teaspoon Dijon mustard
• 1 teaspoon kosher salt
• 0.5 teaspoon black pepper
• 0.5 teaspoon garlic powder
• 0.25 teaspoon red pepper flakes
• 1 teaspoon fresh thyme, chopped
• 1 teaspoon brown sugar

Steps

1. Slice the 3 yellow onions into 1/4-inch-thick rings, separating them as you go.
2. Heat 1 tablespoon of olive oil in a 12-inch skillet over medium-low heat.
3. Add the sliced onions and a pinch of salt to the skillet, stirring occasionally.
4. While onions cook, combine the 1 lb ground beef in a small bowl with 1/2 teaspoon kosher salt, 1/4 teaspoon black pepper, 1/2 teaspoon garlic powder, 1/4 teaspoon red pepper flakes, and 1 teaspoon fresh thyme. Mix gently with your hands until just combined, being careful not to overwork the meat.
5. After 30 minutes of onion cooking, push the caramelized onions to the side of the skillet. Increase heat to medium-high and add 2 tablespoons butter to the empty space.
6. Once butter is melted and foaming, add the seasoned ground beef to the skillet, breaking it into small pieces with a wooden spoon and cooking until no pink remains, about 5–7 minutes.
7. Once beef is cooked through, stir in the caramelized onions to combine evenly with the meat.
8. In a small bowl, whisk together 2 tablespoons balsamic vinegar, 1 tablespoon Worcestershire sauce, 1 teaspoon Dijon mustard, and 1 teaspoon brown sugar until the sugar dissolves.
9. Pour the glaze into the skillet and stir for about 1 minute, coating the beef and onion mixture evenly. Remove from heat.
10. Toast the 8 slices of bread until golden brown, about 2–3 minutes per side in a separate skillet or toaster.
11. Place 4 of the toasted bread slices on a clean work surface. Top each with a generous portion of the beef and caramelized onion mixture.
12. Top each beef-covered slice with 1 slice of sharp cheddar cheese, allowing it to soften slightly from the heat of the filling.
13. Place the remaining 4 toasted bread slices on top, creating 4 sandwiches. Serve immediately while the cheese is still warm and slightly melted.