Pennsylvania Dutch Bread and Potato Filling - Martin's Famous Potato Rolls and Bread

Ingredients

• 2 Pounds Potatoes, peeled and cubed
• 5 Tablespoons Butter, divided
• 3/4 Cup Milk
• 1/2 Cup Chicken Stock
• 1 Cup Yellow Onion, chopped
• 1 1/2 Cups Celery, leaves and stalks, finely chopped
• 1 Pinch Salt and Pepper, to taste
• 5 Slices Martin’s Old-Fashioned Real Butter Bread, torn into pieces
• 2 Tablespoons Fresh Parsley, chopped
• 2 Teaspoons Fresh Sage, chopped
• 1 Egg, beaten (optional)