Slice zucchini into 5mm rounds and coat with potato starch. Mince onion and microwave for 1 minute at 600W, then cool. Wrap tofu in paper towels to remove excess water.

Remove skin and bones from salmon and finely chop.

In a bowl, crumble tofu and mix with salmon, onion, potato starch, salt, and pepper.

In a separate bowl, combine mustard and soy sauce to make the mustard sauce.

Place the salmon mixture on top of zucchini slices, sandwich with another zucchini slice, and press gently.

Heat salad oil in a pan, arrange the zucchini sandwiches, and cook over medium heat until browned. Add sake, cover, and steam for 5 minutes. Flip and cook until the other side is browned.