Wash potatoes and prick with a fork. Bake at 425 for 1 hour or until soft throughout. Let cool a few minutes for easier handling. Cut potatoes in 1/2 or cut a small slice off tops.

Carefully Scoop out the potato from the skin, being careful not to damage the skin. Mash cooked potatoes with butter until melted, and add enough salad dressing to get a creamy consistency. Season potato mixture with salt and pepper, onions, and grated cheese if desired, and mix. Spoon potato mixture back into shells and bake at 350 for 30-45 minutes until heated through.