1. For the potato tots, preheat the oven to 180°C.
2. 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.
3. 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.
4. Fill a heavy-bottomed pot or Dutch oven by one-third with rapeseed oil. Heat the oil to 190°C. 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.
5. 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.
6. Serve the tots with the ranch dressing on the side.