Copycat In-N-Out Animal Style Fries

For the fries:

• 32 ounces frozen French fries
• 4 American Cheese Slices

For the onions:

• 1 medium yellow onion, chopped
• 1 TBS butter
• ⅛ teaspoon salt
• ⅛ teaspoon pepper

For the sauce:

• ½ cup mayonnaise (reduced fat or regular)
• 3 TBS ketchup
• 1 teaspoon yellow mustard
• 1 teaspoon white vinegar
• 2 ½  TBS sweet pickle relish
• dash salt
• dash pepper