Remove the bones from the fried mackerel and flake the meat. Thinly slice the lemongrass, shallots, chilies, kaffir lime leaves, green onion, and cilantro. Prepare the chilled rice and egg.

Heat oil in a skillet or wok. Add the lemongrass, shallots, and chilies, and stir-fry until fragrant. Add the mackerel and stir-fry to combine. Add the egg and stir until just set. Add the rice and stir-fry over high heat until the rice is hot and separated.

Season with fish sauce, sugar, and black pepper. Stir to combine. Add the sliced kaffir lime leaves, green onion, and cilantro, and stir just until mixed. Remove from heat.

Serve on a plate with cucumber slices and a wedge of lime.