Baked feta with sumac grapes

Name: Baked feta with sumac & grapes
Ingredients: 1 small red onion halved and sliced; 2 tbsp red wine vinegar; 1 tsp golden caster sugar; 350g red grapes left on the vine in small bunches; 2 tbsp olive oil; 2 tsp sumac; ½ small pack dill chopped; 2 x 250g blocks feta; bread or sesame flatbreads to serve (optional)