Squeeze the tofu to remove excess water. I use 2 small chopping boards and some paper towels. place paper towel under and above the tofu. Sit the tofu on chopping board and press down firmly with the other board to squeeze water out. Don't do it too hard. Some tofus are more watery than others.
(If you have any leftover tofu after this recipe, store covered, in the fridge, immersed in fresh water.)
Place the tofu in a bowl and mash with a fork until it's in small flaky pieces.
Add the spring onion/red onion and watercress. Mix.
Add the mayonnaise. Mix.
Season to taste.
Chill the tofu mayonnaise if liked. It will keep fine for up to 3 days, covered in fridge. Possibly longer.
Prepare as you would any other sandwich. I like to add extra chopped watercress on top of the tofu mayonnaise in the sandwich. Extra iron! Adding a layer of rocket leaves between the bread and tofu mayonnaise is also delicious, adding a peppery hit.