Bring brown rice and water to a boil in a saucepan over high heat. Reduce heat to medium-low; cover and simmer until rice is tender and the liquid is absorbed, 45 to 50 minutes.
Combine red bell pepper, peas, raisins, onion, and olives in a bowl.
Whisk vegetable oil, vinegar, and mustard together in a separate bowl for the balsamic dressing.
Stir brown rice and balsamic dressing into vegetable mixture. Season with salt and black pepper.
Top brown rice and vegetables with feta cheese before serving.
