Prick each potato with a fork.Wrap each potato in a piece of aluminum foil with a slice of onion, butter, salt and pepper. Tightly roll the each potato up in the aluminum foil, tucking the edges as you go.
Set aside and repeat with the remaining potatoes.Layer the potatoes in the bottom of your crock pot. You can stack them if need be. Pour a ½ cup water into the bottom of the crock pot.
Cook potatoes on high for 5 to 6 hours or until fork tender.Rotate the potatoes half way through the cooking if you have stacked them. For a single layer turn the potatoes at the half way mark.
Turn the crock pot on to the warm setting to keep the potatoes hot while serving.Serve hot with your favorite baked potato toppings, like butter, sour cream, cheese, bacon bits, and chives.