Vegan potato cake with chickpea curry

Name: Vegan potato cake with chickpea curry
Ingredients: 500g/1lb 2oz cooked chickpeas, cooking liquor reserved, or 2 x 400g tins of chickpeas, drained; 50ml/2fl oz vegetable oil; 1 tsp yellow mustard seeds; pinch salt; 1 tsp chilli powder; 75g/2½oz piece root ginger, peeled and shredded; 5 ripe tomatoes, cut into chunks; small bunch curry leaves; small handful coriander leaves; 3 baked potatoes; 1 tbsp plain flour, plus extra for dusting; ½ tsp salt; 4 tsp garam masala; 2 red chillies, seeds removed and finely diced; small handful coriander, chopped; flour, for shaping; vegetable oil, for frying