Steps
- Lightly toast 6 slices of rye bread. Shape 450g of ground chuck into 3 flat rectangles about 1in / 2.5cm longer than the bread. Season with salt and pepper.
- Melt one-fifth of ½ cup plus 2 tablespoons of butter in a large skillet over medium heat. Brown ½ sliced red onion and remove.
- Increase to medium-high heat and melt another fifth of the butter. Cook the patties, turning once, until they reach 160°F / 71°C.
- Spread the remaining butter over the bread exteriors. Spread 3 tablespoons of mayonnaise inside half the slices and put 6 slices of mild cheddar inside all the slices.
- Sandwich the patties and onions between the bread. Cook in a clean skillet over medium heat, turning, until both sides are golden and the cheese melts.