Drain the cucumber salad well. Put sprouts in a sieve, rinse with cold water and drain well. Peel onions and cut into fine rings. Remove the mackerel flesh from the skin and pluck.
Mix mackerel, sprouts and onions loosely.
Peel and finely chop the garlic. Wash the dill, shake dry and chop finely. Cut the avocado in half lengthwise and remove the core. Remove the flesh from the skin with a tablespoon, dice finely and crush with a fork.
Mix avocado with garlic, dill, mayonnaise, mustard and lemon juice. Season to taste with salt and pepper.
If necessary, cut the bread roll in the middle and toast the halves. First spread the avocado spread, then cucumber salad and mackerel mix on the halves.