Mix sliced beef with sake, soy sauce, and cornstarch in a bowl. Let it marinate for 5 minutes.

Prepare vegetables: remove seeds and pith from bitter melon, slice into 8mm pieces; chop red bell pepper; cube fried tofu into 3cm pieces.

Heat sesame oil in a frying pan over medium heat. Add marinated beef and stir-fry until the beef changes color.

Add prepared vegetables and tofu to the pan. Stir-fry until everything is cooked through. Add mirin, sugar, and soy sauce, then cook until the liquid evaporates.