Split the soda farls and put them in a toaster or under the grill to toast on both sides. Meanwhile, beat together the eggs and milk with a good sprinkling of salt and some black pepper. Grate the cheese.
Heat the butter in a small-to-medium-sized frying pan (preferably non-stick) and when melted add the tomatoes and cook them for about 3 minutes over a fairly high heat, shaking the pan occasionally until they start to soften and the skins split.
Pour the eggs into the pan and leave them undisturbed for about 10 seconds. Stir and leave to settle again, then stir again until the eggs are almost scrambled. Sprinkle in the cheese, roughly chop and add the watercress, then take from the heat. Butter the farls and put on serving plates, pile the scramble on top and serve straight away.