Prepare Ingredients: Fillet the sea bream into three pieces, removing any bones and skin. Mince the raw tuna and chicken separately.
Make Bonito Broth: Prepare the bonito broth as per your usual method.
Steam Ingredients: Steam the chicken, sea bream, broad beans, and Japanese mustard spinach until fully cooked.
Create Paste: Make a paste from the steamed chicken, sea bream, beans, and spinach.
Shape Tuna: Shape the minced tuna into a flat cube and slice it into thin pieces.
Prepare Broth Mixture: Mix the bonito broth with corn starch water.
Shape into Nigiri: Shape the prepared pastes and tuna slices into nigiri sushi pieces.
Serve: Arrange the nigiri sushi on a plate and serve immediately.