Mom's Fabulous Chicken Pot Pie with Biscuit Crust

INGREDIENTS

• 1/4 cup butter
• 1 small onion, chopped
• 3 celery ribs, chopped
• 3 carrots, chopped
• 2/3 cup frozen peas
• 3 tablespoons chopped fresh parsley
• 1/4 teaspoon dried thyme
• 1/4 cup all-purpose flour
• 2 cups lower-sodium chicken broth
• 2/3 cup half-and-half cream
• salt and ground black pepper to taste
• 3 cups cooked chicken, cut into bite-size pieces
• 1 (16.3 ounce) can refrigerated flaky-style biscuits (such as Pillsbury Grands®)
• 1 egg yolk, beaten
• 1 tablespoon water