Cut potatoes in half. Scoop out insides of potatoes, leaving 1⁄8-inch shells. Place insides of potatoes in KitchenAid® Stand Mixer bowl. Attach bowl and flat beater to the KitchenAid® Stand Mixer. Turn to Speed 2 and mix about 1 minute. Add sour cream, margarine, garlic salt and thyme. Turn to Speed 2 and mix about 30 seconds. Turn to STIR Speed and add onions, mixing just until blended.
Spoon or pipe potato mixture into potato shells. Place filled shells in shallow baking dish. Bake at 375°F for 20 to 25 minutes, or until thoroughly heated. Sprinkle with cheese and paprika, if desired. Bake 5 minutes longer, or until cheese is melted. Serve warm.