Step 1 Mackerel thawed, gutted, washed and cut into portions. Add salt and pepper to taste. Step 2 Chop the onion rings, tomatoes -sliced. Step 3 Pieces of fish to put in the ovenproof dish, alternating with onion and tomato. Drizzle with vegetable oil, to grease with mayonnaise. Step 4 Bake in the oven at a temperature of 200 deg. about 30 min. Garnish with greens.