Make the pickled onions at least 4 hrs before serving. Combine the sugar and 100ml boiling water in a non-metallic heatproof bowl. Stir in the vinegar and salt. Add the onions and press down with a spoon to submerge. Cover and leave to pickle for at least 4 hours, or chill overnight.

Heat the oven to 200C/180C fan/gas 6. Finely slice the potatoes at regular intervals along their length without cutting through. Transfer to a baking tray, drizzle with the oil, season, add the garlic and roast for 1 hr-1 hr 20 mins, tossing occasionally, until tender and golden.

Whisk the soured cream with most of the dill, then season. Transfer the potatoes to a serving plate and dollop a teaspoon of the soured cream mixture over each. Drain the pickled onions (use the pickling liquid for salad dressings, if you like) and add a few pieces to each potato. Scatter with the remaining dill.