Pierce potatoes in several places with fork. Microwave on HIGH 16 to 18 min. or until potatoes are tender, turning and rotating potatoes every 6 min. Cool slightly.
Heat oven to 375ºF. Cut potatoes lengthwise in half; scoop out flesh into large bowl, leaving 1/4-inch-thick shell in each potato half.
Mix sour cream, dry seasonings and 1/2 cup dressing until blended. Add to potato flesh; mix well.
Spoon into 13x9-inch baking dish sprayed with cooking spray.
Crumble 6 bacon slices. Add to potato mixture along with the VELVEETA and 1/2 the onions; mix lightly.
Cut 9 potato shells crosswise in half; place, peel sides up, over potato mixture in baking dish. Discard remaining shells. Brush potato peels with remaining dressing.
Bake 20 min. Sprinkle potato shells with cheddar; bake 5 min. or until melted. Crumble remaining bacon slices; sprinkle over casserole along with the remaining onions.