Slice the beef into very thin strips. Next, slice the onions into rings. Sauté the beef and onions in a very hot skillet with oil until golden brown. Season with salt and pepper, remove from the pan, and set aside.
Sauté the bell peppers in the pan with the beef and onion drippings.
Chop the tomatoes and cucumbers into small pieces.
Fill the whole wheat pita pockets with the beef and onions mixture, followed by the bell peppers, tomatoes, and cucumbers. Sprinkle the feta on top and enjoy!