Blanch spinach briefly, drain, cool, and squeeze out water. Mix with soy sauce and bonito flakes. Set aside.

Heat a frying pan, add oil and eggs, and cook eggs over easy. Cover with a lid and cook a minute, turn over, and cook another minute without the lid.

Place a sheet of Nori on a piece of plastic, a corner pointing at you. Put 1/2 cup of the rice in the middle and flatten. Put half of the spinach, 1 egg, and half of the beef on the rice. Cover with another 1/2 cup of rice, and flatten again without squashing too much. Bring all the corners of Nori to the center, and wrap the whole thing tightly with plastic. With the plastic still on, cut in half.

Repeat with the remaining ingredients.