Mix the buckwheat flour with the egg and a splash of water until you have a smooth batter. Whisk out any lumps. Rinse the cucumber and half the fennel, then slice both vegetables into very thin slices or strips. Flake the smoked mackerel into large pieces and carefully check for any remaining bones.
Heat a non-stick frying pan over medium-high heat. Pour small portions of batter into the hot pan to form multiple pancakes. Cook the pancakes for about two to three minutes per side, until the edges are dry and both sides are light golden brown. Remove from the pan.
Divide the warm pancakes among the plates. Top them first with the thinly sliced cucumber and fennel strips. Then distribute the flaked smoked mackerel evenly over the vegetables on the pancakes.
Serve the dish immediately while the pancakes are still hot.