Chicken Pot Pie

Name: Chicken Pot Pie
Prep time: 20 min
Cook time: 25 min
Total time: 45 min
Cuisine: European, British
Ingredients: 1 tablespoon olive oil; 1 large onion; 500 grams chicken; 400 grams button mushrooms; 2 large carrots; 2 celery stalks; 2 tablespoons butter; 40 grams flour; 250 ml chicken stock; 200 ml milk; 1 1/2 teaspoon garlic powder; 1 1/2 teaspoon dried thyme; 1 teaspoon dijon mustard; 50 grams parmesan cheese; 1 puff pastry sheet; 1 egg - whisked; salt and pepper to taste; sesame seeds to finish