Chicken Pot Pie Burgers

Name: Chicken Pot Pie Burgers
Ingredients: 4 tablespoons unsalted butter; 2 small ribs celery, finely chopped; 1 small carrot, grated; 1 medium shallot, finely chopped; 1/4 teaspoon powdered chicken bouillon; 1/4 teaspoon celery seeds; Kosher salt and freshly ground black pepper; 2 tablespoons all-purpose flour; 1/4 teaspoon garlic powder; 1 cup plus 2 tablespoons whole milk; 1/3 cup frozen peas; 1/4 cup panko breadcrumbs; 1 pound ground chicken; Nonstick cooking spray, for the skillet and patties; 4 hamburger buns, lightly toasted