Patty Melt

Name: Patty Melt
Cook time: 35 min
Total time: 35 min
Cuisine: American
Ingredients: 6 tablespoons unsalted butter, divided; 2 medium yellow onions, thinly sliced (about 4 cups); 1 garlic clove, thinly sliced; 1 1/2 teaspoons kosher salt, divided; Water, as needed; 2 teaspoons apple cider vinegar; 1 teaspoon Worcestershire sauce; 1 pound ground beef, such as 85/15 ground round; 1/2 teaspoon black pepper; 1 tablespoon canola oil; 2 tablespoons yellow mustard, divided; 8 (1-ounce) seeded rye bread slices, lightly toasted; 8 (1-ounce) Swiss cheese slices, cut in half