Sourdough Cheeseburgers

Ingredients (9)

• 3 tablespoons mayonnaise
• 1 tablespoon ketchup
• 1 tablespoon sweet pickle relish
• 1/2 pound ground beef
• Salt and pepper to taste
• 1 small onion, sliced and separated into rings
• 4 tablespoons butter, divided
• 4 slices sourdough bread
• 4 slices Swiss cheese