HEAT greased grill to medium heat.
BRUSH onions and cut sides of peach halves with 1 Tbsp. dressing. Grill peaches 6 min., turning occasionally and adding onions to the grill after 2 min. Cool slightly. Slice peaches and separate onions into rings.
COMBINE spinach and arugula; place on platter. Top with peaches, onions, cheese and nuts.
DRIZZLE with remaining dressing.