DO-AHEAD TIP: Marinate steak for about 30 minutes (see recipe).
Place peeled kiwis in a large zipper-topped plastic bag; seal bag and mash kiwis to a pulp; add cranberry juice and blend well.
Pierce steak with a fork numerous times and place in the bag along with lemon juice and salt and pepper; seal bag and turn to coat; marinate at room temperature for about 30 minutes (no longer, or meat may turn mushy!).
Preheat outdoor or indoor grill or oven broiler. Grill/broil steak to desired level of doneness.
Serve steaks on a bed of chopped spinach.