HEAT oven to 400°F.
PIERCE potatoes in several places with fork. Bake 1 hour 15 min. or until tender. Cool slightly.
CUT potatoes lengthwise in half; scoop out centers into medium bowl, leaving 1/4-inch-thick shells. Mash potato centers. Add Neufchatel and dressing; mix well. Stir in spinach, onions, pepper and 1 cup cheddar.
SPOON potato mixture into shells; sprinkle with remaining cheddar. Place on baking sheet.
BAKE 15 min. or until heated through.