For the potato tots: Preheat the oven to 350 degrees F.
Poke holes in the potatoes, then set on a baking sheet. Bake until the potatoes are almost completely cooked, 40 minutes. Remove from the oven and let cool completely before peeling.
Grate the peeled potatoes and the onion in a food processor fitted with the grating blade. Add to a bowl and stir in the flour and some salt and pepper. Mix well to combine. Form the mixture into small cylinders about 1 1/2 inches long and 1/2 inch thick. Place onto a parchment-lined baking sheet.
Fill a heavy-bottomed pot or Dutch oven by one-third with canola oil. Heat the oil to 375 degrees F. In batches, fry the tots until golden brown, 1 to 2 minutes. Remove to a paper-towel-lined baking sheet and sprinkle with salt while still warm.
For the ranch dressing: Stir together the buttermilk, mayonnaise, Greek yogurt, garlic, chives, dill, mustard, lemon juice and hot sauce in a small bowl. Season to taste with salt and pepper. Cover and chill until ready to use.
Serve the tots with the ranch dressing on the side.