Preheat your oven to 375.
In a medium bowl, combine the tuna, mayo, and sour cream. Spread onto a 9" pie dish. Top with the sliced onion.
Dice the potatoes and add to a large pot and cover with water. Boil until fork tender, about 7-10 minutes. Drain and return to pot along with half and half and butter. Mash until smooth. Season with salt and pepper.