Rinse the quinoa under cold water and cook in plenty of salted water for twelve to fifteen minutes until tender, until the germs release. Drain and let steam dry and cool in a sieve.
Cut the tomatoes and cucumber into small cubes and finely chop the red onion. Coarsely chop a generous bunch of parsley and some mint. Mix everything into the cooled quinoa with two tablespoons of olive oil, the juice of half a lemon, salt, and pepper. A true tabbouleh consists mostly of herbs and vegetables, with the grain playing a supporting role, which fits perfectly with a limited amount of slow carbohydrates.
Pat the mackerel fillet dry and season the skin side with salt. Heat the remaining olive oil in a frying pan until very hot and place the fish skin-side down in the pan. Press down gently for the first few seconds so the skin stays flat, and fry for three minutes until crispy.
Turn the mackerel over and immediately remove the pan from the heat; the residual heat will finish cooking it in about a minute. Serve the fish on top of the tabbouleh with a lemon wedge. Mackerel is one of the oiliest fish species, providing a good amount of omega-3.