South Indian fish curry with chickpeas

Name: South Indian fish curry with chickpeas
Ingredients: 1 tsp rapeseed oil; 1 onion halved and sliced; 1 tsp ground turmeric; 1 tsp black mustard seeds; ½ tsp cumin seed; ¼ tsp ground fenugreek; ¼ tsp chilli flakes; ½ thumb-sized piece ginger finely chopped; 1 large garlic clove finely grated; 400g can chopped tomato; 210g can chickpea drained; ½ fish stock cube, crumbled; 2 tbsp tamarind paste (optional); 350g fresh mackerel cut into thick pieces (or use boneless fillets if you prefer); 250g pack cooked brown rice; 25g flaked almond; small bunch coriander chopped