Comforting chicken pot pie pasta dish

Ingredients

• 12 oz penne pasta
• 1 medium onion, diced
• 2 medium carrots, diced
• 2 celery stalks, diced
• 2 cloves garlic, minced
• 1 cup frozen peas
• 3 tbsp unsalted butter
• 1/4 cup all-purpose flour
• 2 cups chicken broth
• 1 cup whole milk
• 1/2 tsp dried thyme
• Salt, to taste
• Black pepper, to taste
• 3 cups cooked chicken, shredded or chopped
• 1 sheet puff pastry, thawed
• 1 egg, beaten