Start by preparing the mackerel, eviscerate and cut into fillets. Season with salt and set aside.
Wash and cut the cucumber into very thin slices with a knife or a mandoline, season with salt and reserve.
Bake the potatoes in boiling water seasoned with salt until they are very soft. Drain the water and mash with a fork.
Wash the tomatoes and cut into small cubes, keeping the juices and seeds. In a pan, heat a tablespoon of olive oil, add the garlic, tomatoes, their juices and a pinch of salt and cook over low heat until the tomatoes are almost dissolved, about 5 minutes, stirring occasionally to prevent from sticking to the bottom of the pan. Add the crushed potatoes, season with pepper and nutmeg freshly grounded and combine well. Simmer over low heat, stirring constantly, until the potato and tomato are well mixed together.
Heat a grill. Brush the skin of the mackerel fillets with olive oil and place, skin side down, on the grill. Turn the fish after two minutes and let cook for another minute.
Drain off the water from the cucumber and season with a little olive oil and a little vinegar. Place some of the cucumber salad in the center of the dish. With the help of two spoons, shape small cakes of potato and tomato mash, like a quenelle, and place also on the dish. Place the fillets on top of the cucumber and serve immediately.