In medium bowl, whisk together soy sauce, mustard, sugar and ginger, set aside. In large, nonstick skillet over high heat, toast sesame seeds until golden brown about 4 minutes, remove from pan and set aside.
In same skillet, sauté onion and garlic in oil until soft about 5 minutes, if desired discard garlic clove halves. Add chicken and soy sauce mixture to skillet. Cover and simmer, turning chicken over once until chicken is cooked through.
Meanwhile, prepare spinach and pepper sauté. To serve, place chicken on platter, pour sauce and vegetables over chicken and sprinkle with sesame seeds. Place sautéed mixture on platter. Garnish with scallion fan.
