Grate the potatoes and tofu together through your food processor; tip them into a large mixing bowl (or sieve the potatoes and tofu directly into the bowl); add seasoning.
Sieve the cornflour onto the potato mix then using a wooden spoon or hard spatula mix the contents of the bowl together using a cutting motion; pour over the tablespoon of melted margarine and add the scallions and chopped tomatoes (if the tomatoes are very watery you may wish to drain some of the juice off first); again mix the ingredients together with a cutting motion.
Knead the mixture slightly into a soft dough, then tip out onto a well floured surface; keeping the dough well floured, roll it to 3/4 inch thickness; cut into rounds, 2 1/2 inches across.
Place the rounds on a greased baking tray; (brush with extra melted margarine and sprinkle tops with sea salt if using) and bake for about 20 - 25 minutes until golden brown (if you wish, you can finish them off under the grill for a few minutes for a crisper top).
You may wish to fry these instead, in which case you are aiming for crisp on the outside, but very soft inside.