Cut the polenta into rectangles, approximately 1½ inches by 3½ inches (3 cm by 7 cm). Place the mackerel fillets in a bowl, season with salt and pepper and stir in 1 tablespoon of chives and 2 tablespoons of olive oil. Blanch the fava beans in boiling water for 1 minute and transfer to a bowl of ice water, reserving the cooking water. Drain, remove the outer skin and place the fava beans in a small mixing bowl.
Using the same water, blanch the tomatoes, drain and peel. Cut the tomatoes open, remove the seeds and dice the pulp. Add the tomatoes to the fava beans and season to taste with salt and pepper and the remaining 2 tablespoons of olive oil.
Heat a grill pan over high heat. Grill the polenta rectangles on both sides and place on serving plates. Spread a little mayonnaise on each polenta rectangle and top with a mackerel fillet and the fava mixture. Garnish with chopped chives if desired and serve.