Chicken kebabs with homemade flatbreads

Name: Chicken kebabs with homemade flatbreads
Ingredients: 300g plain flour, plus extra for dusting; 1 tsp nigella seeds; 1 tsp chilli powder; 2 tbsp olive oil, plus extra for frying; 30g unsalted butter, melted; 1 onion, grated; 1kg chicken mince; 5 garlic cloves, crushed; 1 green chilli, deseeded and finely chopped; 1 tbsp cumin seeds; 1 tbsp ground coriander; 1 tsp chilli flakes; 1 tbsp sumac; 1 tbsp olive oil; 200g red cabbage, finely sliced; 2 tbsp white wine vinegar; 1⁄4 cucumber, finely sliced; 1 Little Gem, shredded; 3 beef tomatoes; 1 red onion, sliced; small handful of parsley; 1 lemon, juiced; 250g Greek yogurt; 1 tsp sumac