Use boneless, skinless half-breasts. Cut crosswise into 1/2-inch strips. Cut onion into wedges, separate into strips. Cut peppers into 1-inch strips. Slice mushrooms. In 12-in. Skillet over med heat, cook onion in oil til tender. Add peppers, increase heat and cook til tender-crisp and lightly browned, stir often. Remove veggies to bowl. In remaining oil over high heat, cook chicken about 5 mins, til it loses pink color and is lightly browned. Add to veggie mixture. Add 2 Tbsp olive oil. Cook mushrooms til tender. Add next 5 ing. Cook about 5 mins to heat through. Return remaining ingredients to skillet; heat through. Stir in arugula just to wilt.