Special equipment: a deep-frying thermometer
For the chipotle ketchup: Put the ketchup, chipotle chile, adobo sauce, cilantro and lime zest in a blender. Blend until smooth.
For the hot potato tots: Preheat the oven to 250 degrees F.
Put the potatoes in a stockpot and cover with cold water. Bring to a boil over medium-high heat and cook until the potatoes are fork tender, 10 to 12 minutes. Drain and let cool.
Grate the potatoes and onion into a bowl, using the large holes on a box grater. Mix in the flour, thyme, egg, 2 teaspoons salt and 1 teaspoon pepper.
Fill a Dutch oven two-thirds full with oil and heat to 375 degrees F.
Scoop about 1 heaping tablespoon of the potato mixture into the palm of your hand. Gently shape into a ball, then flatten with a two-finger pinch and roll between your palms. Repeat with the remaining potato mixture.
Fry a few tots at a time, turning frequently, until golden brown, 3 to 4 minutes. Transfer to a paper-towel-lined plate and sprinkle with salt. Keep warm in the oven. Serve with the chipotle ketchup on the side.