While rice cooks, peel onion and cut off stem top and root bottom. Cut onion in half from stem top down. Place each half onion flat side down on cutting board and cut into 1/4-inch slices horizontally. Cut into 1/4-inch slices vertically then cut into 1/4-inch pieces crosswise. Set aside. Cut coarse green tops from green onions, leaving a little green. Holding onions together, cut off roots, then cut green onions into small pieces; set aside. Cut stems off peppers, then cut each pepper in half lengthwise. Cut out white veins and seeds and discard. Cut each pepper half into 1/4-inch strips. Hold bundle of strips together and cut crosswise into small pieces. Add to onion pile. Trim leaves from celery tops. Slice each celery rib lengthwise from top to bottom in 1/4-inch strips. Holding strips together, chop into small pieces crosswise. Add to prepared vegetables.