How to Make It
Heat grill to high. Once it is hot, clean the grill grate with a wire brush. Just before grilling, oil the grill grate.
Split each chicken breast horizontally (do not cut all the way through); open and pound to a ½-inch thickness.
Season the chicken with the coriander, ½ teaspoon salt, and ¼ teaspoon pepper. Grill, until cooked through, 2 to 3 minutes per side.
Combine the oil, lemon juice, ½ teaspoon salt, and ¼ teaspoon pepper in a large bowl. Add the arugula, radishes, and onion and toss to combine. Serve over the chicken.