Slice the cucumber thinly and lightly salt it.
Julienne the onion and carrot finely.
Drain the oil from the canned tuna and set it aside.
Make the sauce with gochujang (Korean chili paste), sesame oil, sugar, and minced garlic.
Place cucumber, onion, carrot, tuna, and seaweed flakes on top of the rice.
Add the sauce and mix all the ingredients well together.