Chicken Pot Pie

1 Sheet Store-Bought Puff Pastry, thawed
2 Large Carrots, peeled and diced
1 Large Onion, diced
2 Stalks of Celery, diced
1 ½ lb Boneless Skinless Chicken Thighs, cut into 1 inch pieces
3 cups Chicken Stock
1 cup Frozen Pearl Onions, thawed
1 cup Frozen Peas, thawed
1/3 cup Unsalted Butter
1/3 cup All Purpose Flour
½ cup Heavy Cream
Salt and Pepper, to taste
2 Tbsp Olive Oil
1 Tbsp fresh thyme, chopped
1 Tbsp Fresh Parsley, chopped
1 Egg, beaten with 1 tbsp of water to make egg wash