Turkey Pesto Super Sub Sandwiches

Ingredients (10)

• 24 oz. large round loaf of soft bread
• 1 Tbsp. olive oil
• 1 large red onion
• 1/4 cup prepared basil pesto
• 1/4 cup prepared mustard
• 1 lb. roasted turkey slices
• 1 lb. sliced Colby Jack
• 1 lb. sliced Monterey Jack
• 1/2 cup sliced pepperoncinis
• 1/4 cup butter