Toss potatoes, bacon fat, salt, pepper together in small dutch oven. Add thyme and halved red onion. Cover, roast 30 min. Uncover, roast 15 min. Remove from oven, allow to cool at room temp.
Dice cooked onion, set aside.
Fry bacon until crisp, reserving rendered fat. Allow to cool.
Whisk sauce ingredients together.
Remove thyme stems from potatoes, toss with sauce, onion and bacon. Refrigerate at least one hour. Better overnight!