Best Vegetarian Pot Pie

Name: Chickpea Pot Pie
Ingredients: 1 large egg; 1 sheet frozen puff pastry (half of a 17.3-ounce package), thawed; 4 tablespoons unsalted butter; 2 medium to large yellow onions, finely diced; 1 pound carrots, peeled and finely diced; 1 pound (about 6 stalks) celery, finely diced; 2 teaspoons kosher salt, plus more to taste; 3 garlic cloves; 7 tablespoons all-purpose flour; 1 1/2 cups water, warm; 1 1/2 teaspoons vegetable-based Better Than Bouillon; 2 tablespoons white miso paste; 2 1/2 cups whole milk, warm; 1/4 teaspoon freshly ground black pepper, plus more to taste; 1 cup frozen peas; 2 (15.5-ounce) cans chickpeas, drained; 1/4 cup finely chopped parsley