Preheat the oven to 180°C.
Peel the sweet potatoes then grate coarsely.
Carefully remove the skin from the mackerel then flake the fish meat.
Mix the grated sweet potatoes and mackerel flakes in a large bowl.
Add the chopped herbs, lemon zest, lemon juice and flour, then season. Mix well.
Shape the mixture into 6- 8 cakes and place these on a non stick baking tray, brushed with the oil. Use the remaining oil to brush the top of the fishcakes.
Bake for 20-25 minutes until golden.
Serve warm or at room temperature.