Greek Pita Pockets

- In a large skillet over high heat, cook ground beef and onion until all pan juices evaporate and beef is well browned. Skim off any excess fat. - In a cup, stir water and flour until blended. Stir flour mixture into meat mixture with salt, oregano, and pepper; heat to boiling. - Reduce heat to medium and cook, stirring constantly, until mixture is thickened. - Remove skillet from heat; stir in tomato, olives, and vinegar; then stir in cheese. - To serve, spoon ground beef mixture into halved pockets .