Step 1

In a small bowl, stir together the mayonnaise, horseradish and lemon zest.

Step 2

In another small bowl, combine the cucumber, radish, shallot, if using, and lemon juice. Season with a pinch of salt, if desired, and stir to combine.

Step 3

Lay the bread on a plate. Spread the horseradish mayonnaise on both slices. Pop open the can of mackerel and, using a fork, transfer the fish fillets to the bread, slightly smashing them into the bread, if desired. Top with the lemony vegetables, parsley and cracked black pepper, and serve.