Chicken Pot Pie with Leeks and Mushrooms

Ingredients

• 2 tbsp oil
• 6 boneless, skinless chicken breasts
• 1 tsp poultry seasoning
• 2 cups carrots, peeled and sliced
• 3 stalks celery, cut in half lengthwise and sliced
• 3 leeks, light green and white part, sliced
• 1 cup cremini mushrooms, sliced
• 3 tbsp butter
• 3 tbsp flour
• 2 tbsp Dijon mustard
• 1/2 cup white wine
• 2 cups low-sodium chicken broth
• 1 cup 35% (whipping) cream
• salt, to taste
• pepper, to taste
• 1 sheet puff pastry, thawed
• 1 egg(s), beaten