1 In a food processor, coarsely chop the scallions and set aside. In the same work bowl, coarsely chop the water chestnuts.
2 In a medium bowl, combine the beef with half the garlic, half the scallions, all of the water chestnuts, the orange zest (if using) and red pepper flakes. Form the mixture into four equal patties about 3/8 inch thick.
3 In a large skillet, warm the vegetable and sesame oils over medium-high heat until hot but not smoking. Add the hamburgers and cook for about 3 minutes per side for medium-rare; 4 minutes per side for medium; 5 minutes per side for well-done.
4 Remove the hamburgers to a plate and cover loosely to keep warm. Reduce the heat under the skillet to medium and add the remaining garlic and scallions, the orange juice concentrate, soy sauce, honey, tomato paste and pepper. Bring to a boil.
5 Meanwhile, in a small bowl or measuring cup, combine the chicken broth and cornstarch. When the mixture in the skillet comes to a boil, stir in the cornstarch mixture and cook, stirring, until slightly thickened, 2 to 3 minutes.