1. Preheat grill to medium-high heat.
2. Combine first 4 ingredients in a small bowl. Divide mixture in half. Combine half of balsamic mixture with 1 tablespoon maple syrup. Reserve 2 tablespoons maple syrup mixture. Brush remaining maple syrup mixture evenly over pork; sprinkle pork with salt and pepper. Place pork on a grill rack coated with cooking spray; grill 16 minutes or until desired degree of doneness, turning occasionally and brushing with reserved maple syrup mixture. Remove pork from grill, and let stand 10 minutes before slicing.
3. Coat cut sides of nectarines with cooking spray; brush with remaining 1 tablespoon maple syrup. Place, cut sides down, on grill rack; grill 5 minutes or until well marked and tender. Remove from grill; slice into 1/2-inch wedges.
4. Combine remaining half of balsamic mixture, greens, and onion in a large bowl, tossing to coat. Divide mixture among 4 plates; top with pork and nectarines.
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