HEAT oven to 350°F.
CUT 1/2-inch-thick slice from top of each onion; discard top. Trim bottoms so onions stand upright. Scoop out centers of onions, leaving 1/2-inch-thick shells. Stand onion shells in pie plate. Add 1/2 cup water; cover.
PLACE removed onion centers in blender container. Add dressing and mustard; blend until smooth. Place chicken in 13x9-inch baking dish; brush with dressing mixture.
BAKE chicken 45 min.; baste with dressing mixture. Return to oven with onion shells. Bake 20 min. Meanwhile, combine cream cheese, spinach, nuts and cracker crumbs.
SPOON spinach mixture into onion shells. Bake 25 min. or until chicken is done (165ºF) and onions are tender. Serve chicken with onions.