This behemoth, double patty delight poses a welcome challenge for both burger and cheese lovers alike.
To start, whisk mayo and mustard in a small bowl. Refrigerate.
Heat olive oil in a large sauté pan over medium/high heat. Add butter and melt. Add the onions and stir occasionally for about 30 minutes. Onions will become a dark brown color. Tip: If onions begin to burn, occasionally stir in a small amount of water while cooking. Once browned, keep warm in pan on low heat.