Copycat In-N-Out Animal Style Fries

For the fries:

• 32 ounces French Fries frozen
• 4 American cheese slices

For the onions:

• 1 medium yellow onion chopped
• 1 TBS butter
• ⅛ teaspoon salt
• ⅛ teaspoon black 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 pickle relish sweet
• dash salt
• dash black pepper