Place chicken between 2 sheets of heavy-duty plastic wrap; flatten to 1/2-inch thickness using the flat side of a meat mallet or rolling pin.
Stir together mayonnaise, brown mustard, and next 3 ingredients. Sprinkle chicken evenly with salt and pepper; coat chicken evenly with mayonnaise mixture.
Grill, covered with grill lid, over medium-high heat (350° to 400°) 4 minutes on each side or until done.
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