Julia Child's Pan-Fried Thin Burger

INGREDIENTS

• 1 red onion, sliced very thin
• 1 to 1 1/4 pounds fresh ground beef (preferably 15 to 20 percent fat)
• 1 teaspoon butter or mild olive oil
• Condiments
• Butter, at room temperature
• Salt
• Iceberg lettuce leaves
• Potato chips
• Freshly ground pepper
• Ketchup
• Thin cheddar or Swiss cheese slices
• Mayonnaise
• Small whole pickles
• 1 tomato, sliced thin
• Salsa
• 1 tablespoon minced shallots
• Dijon mustard
• Bacon strips, cooked crisp