Combine the mirin, sesame oil, and vinegar in a shallow dish. Add the tofu and marinate for 15 minutes. Then turn, and marinate for 15 more minutes.
Meanwhile, bring a pot of water to a boil and set up an ice bath. Blanch the broccoli, transfer to the ice bath, and drain.
Preheat a grill pan, preferably cast-iron, over high heat. Add the tofu and cook, turning once; do not discard the marinade.
Heat the vegetable oil in a skillet over medium heat. Add the onion and garlic and cook just until soft. Then, add the bell pepper, broccoli, tofu, and marinade, and stir-fry, tossing frequently, for 3-5 minutes. Chill in the refrigerator. Sprinkle on the sesame seeds before serving.