Bring a small pot with rice and 1 ½ cups of water to a boil. Reduce to a simmer, cover, and cook until rice is tender, 20 minutes. Thoroughly rinse produce and pat dry. Core and cut red and green bell peppers into 1” dice. Trim and thinly slice top 2” of green part of green onion for garnish and slice remaining green onion into 1” lengths. Halve grape tomatoes. Rinse chicken breasts, pat dry, and, on a separate cutting board, cut into 1½” dice. Season chicken with a pinch of salt and pepper and reserve.