To make Tuna Fish Cakes, start by draining the canned tuna.
While the rice is cooking, prepare the eel by grilling or frying it.
Put the tomato paste, olives, and butter into a blender or food processor and whizz until smooth.
Put the oats in the blender or food processor, slightly broken up.
Coat the fillets with the butter mixture while grilling.
Heat a large skillet over medium-high heat and add the fish cakes.
Season well. Add the spring onions, stir and cook for a couple more minutes until the onions are soft.
Dip in the flour and coat each fishcake. Dip in the egg and then coat with crumbs. Repeat this process until you have 24 fishcakes. Chill the fish cakes for one hour.
Brush the mackerel fillets with harissa paste and pan-fry them skin-side for 3 to 4 minutes. Then turn the fish and cook the other side for 30 seconds more, until cooked through.
Place the chopped parsley over the potato mixture, top with the fish.
Serving Suggestions: Serve with lemon wedges. This recipe goes well with seeded flatbreads.