Cut the kabocha squash into 5-mm thick wedges (kushigata-giri) and slice the carrot into 5-mm thick rounds (wa-giri). Remove the cap and the seeds of the green bell peppers and quarter lengthwise. Cut the tuna into 2-cm wide strips and sprinkle with sake.
Bring the dashi stock and the (A) seasonings to a boil in a sauce pan. Heat half of the oil in a non-stick frying pan and sauté the carrot, kabocha squash, and green bell peppers until browned. In another frying pan, heat the remaining oil and grill both sides of the tuna strips.
Marinate half of the grilled tuna in the (2) ingredients with the vegetables. Place on a plate and add the remaining marinade.