In a skillet, heat oil done on the two sides. Add onion and cook until mellowed, 3 to 4 minutes. Add tomato stick; blend 1 second. Add pureed tomatoes, sugar, vinegar, Worcestershire sauce, and 1 cup water. Stew 10 minutes. Add brisket; toss to warm through. Serve on toasted buns, with cut red onion and cilantro.