To make the tartare sauce, mix the dill, capers, gherkins, lemon zest, mayo and horseradish together and set aside. Can be made a day ahead and chilled.

Heat the grill to high. Grill the bacon for 8-10 mins, turning halfway, until crisp. Remove from the tray and keep warm. Lay the mackerel skin-side up in the bacon fat, drizzle the skin with a little olive oil and season with salt, then grill for 5 mins until the skin is crisp and the flesh can be flaked.

Slice the eggs and lightly toast the bread, then spread the tartare sauce over four of the slices. Lay the bacon, egg and sliced tomato on two of the mayo-covered slices, then lay the warm mackerel and lettuce on the other two. Stack the mackerel slices on the bacon slices and top off with the final slice of toasted bread on each, press down gently and carefully halve the sandwiches and use cocktail sticks to hold them together, if you like. Serve immediately for four as a light lunch or a big lunch for two.