Peel onions and cut them into fine rings. Drain cucumber salad well. Put sprouts in a sieve, rinse with cold water and drain well. Remove the mackerel flesh from the skin and tear it up. Mix mackerel, sprouts and onion rings loosely
Peel and finely chop the garlic. Wash the dill, shake dry and chop finely. Put something aside for garnishing. Put garlic, mayonnaise, mustard, lemon juice and dill in a wide bowl, mix
Cut the avocado in half, remove the seeds. Remove the flesh from the skin, dice finely and crush with a fork. Stir into the mayo mixture. Season to taste with salt and pepper
Cut the bread roll at the marked point, toast. Spread the spread on the cut surfaces of the halves. Place cucumber salad on top. Spread the mackerel mixture loosely. Garnish with remaining dill and arrange on boards