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Put the red onion in a bowl and add the vinegar, sugar and a pinch of salt. Massage everything together with your fingers for 1 min – this will slightly pickle the onions. Heat oven to 200C/180C fan/gas 6.
Scrunch up a sheet of baking parchment and use to line a gratin dish. Add the grapes, drizzle over 1 tbsp olive oil, 1 tsp sumac, most of the dill and some seasoning, then toss everything to coat. Nestle the blocks of feta in among the grapes, then scatter over the onions and any of the pickling vinegar, the remaining oil and sumac. You can now leave in the fridge for up to 24 hrs, or bake straight away.
Bake for 30-35 mins until the feta is browning and the grapes look juicy. Scatter over the remaining dill, then serve in the middle of the table with bread for scooping up the cheese, grapes and juices.