Foryoueats: The Best Patty Melt

Ingredients

• 6 tablespoons butter
• 2 slices rye bread
• 4 slices American cheese, torn into large pieces
• 4 slices Swiss cheese, torn into large pieces
• Kosher salt
• Freshly ground black pepper
• 1/2 pound freshly ground chuck, formed into two 4-ounce patties, roughly the size and shape of 1 slice bread
• 1 large onion, split in half, sliced thin from pole to pole (about 2 cups)
• 1/2 cup water