Chicken Pot Pie Keeper

Ingredients

• 1 (15 ounce) package double-crust pie pastry, thawed
• 1 tablespoon butter
• 4 celery ribs with leaves, diced
• 2 leeks, diced
• 1/2 (10 ounce) package frozen peas
• 2 carrots, peeled and diced
• 2 red potatoes, diced
• 1/2 white onion, diced
• 1 (2 to 3 pound) rotisserie chicken, meat removed and shredded
• 2 cups milk
• 1 tablespoon all-purpose flour, or more if needed
• 1 teaspoon ground black pepper
• 1 teaspoon ground white pepper
• 1/2 teaspoon salt