Thinly slice cucumber and carrot into julienne strips.
Slice the onion thinly and soak in cold water to remove its pungency.
Cut the seaweed into small pieces.
Drain the oil from the tuna using a strainer.
Place rice in a bowl and top with the prepared vegetables, tuna, seaweed, sesame seeds, and gochujang in that order.
Drizzle with sesame oil and mix everything together before serving.