The Best Patty Melt

Name: The Best Patty Melt
Prep time: 5 min
Cook time: 35 min
Total time: 40 min
Cuisine: American
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