Copycat In-N-Out Animal Style Burger | Aunt Millie's

Ingredients

• 1 package Bakehouse Artisan Style Potato Slider Buns
• 1 tablespoon butter
• 1 tablespoon + 1 teaspoon oil, divided
• 3 cups finely chopped yellow onions
• 3/4 teaspoon salt, divided
• 1/2 teaspoon black pepper
• 4 tablespoons water
• 1 pound ground beef
• 3 tablespoons yellow mustard
• 6 slices American cheese
• 1/2 cup mayonnaise
• 1 1/2 tablespoons ketchup
• 1 tablespoon pickle relish
• 1 teaspoon sugar
• 3/4 teaspoon white vinegar
• Shredded lettuce