Gather ground beef, salt, pepper, sliced onions, butter, rye bread, and sliced cheese
Form the ground beef into thin patties slightly larger than the bread
Season both sides of the patties with salt and pepper
Cook the onions in a skillet with butter over medium heat until soft and browned
Cook the patties in a skillet over medium-high heat until browned and cooked through
Place cheese on top of each patty and let it melt
Butter one side of each bread slice
Place one slice of bread butter-side down in a skillet
Add the patty, onions, and another slice of cheese
Top with the second slice of bread, butter-side up
Cook until the bread is golden brown
Flip carefully and cook the other side until golden brown and the cheese is melted
Remove from the skillet and serve hot