Ingredients
Method
- Divide the beef into two loose balls and heat a cast-iron skillet over medium-high heat.
- Set the beef in the skillet, mound onion over each ball and smash into broad thin patties; season with salt and pepper.
- Cook for 3 minutes, scrape and flip onion-side down and cook 2 to 3 minutes more to 160°F.
- Layer cheese, patties and mustard between bread slices and butter the outsides.
- Reduce heat to medium-low and griddle for 2 to 3 minutes per side until golden and melted.