Gather and prepare your ingredients.
Place the red onion into a bowl, juice the lemon over and toss to coat. Set aside to stand while you make the rest.
In a small bowl, stir together the red pesto and balsamic vinegar.
Using either a pastry brush or clean hands lightly coat the mackerel fillets in olive oil and sprinkle with salt and pepper.
Heat a frying pan over high heat, then turn down to medium high (this ensures crispy skin) and place the mackerel fillets in, skin side down. Cook for 4-5 minutes, until the flesh is turning opaque and the skin is golden and crispy. Then flip over and cook for 1-2 minutes on the flesh side. Remove from the pan and allow to cool slightly. Spoon over half of the red pesto balsamic mixture and spread all over the cooked fish.
Put the bread into the toaster. Once done, rub all over with a clove of garlic, then slice into bite-sized pieces.
Arrange the rocket on serving plates and top with the mackerel, tomatoes, avocado and pickled onions. Dollop the rest of the pesto balsamic on top and season with salt and pepper.