Combine mustard and soy sauce in a shallow bowl; set aside.
In a small skillet, toast onion over medium heat until lightly browned, about 3 minutes. Pour into a shallow bowl. Add crumbs and bouillon; mix well. Dip chicken in mustard mixture, then coat with crumb mixture.
Place on a rack over a greased baking sheet. Bake, uncovered, at 350° for 25 minutes or until juices run clear.