Before you start, put your oven on its lowest setting, ready to keep things warm. Peel the potatoes, grate 2 of them, then set aside. Cut the other 2 into large chunks, then boil for 10-15 mins or until tender. Meanwhile, squeeze as much of the liquid from the grated potatoes as you can using a clean tea towel. Mash the boiled potatoes, then mix with the grated potato, spring onions and flour.
Whisk the egg white in a large bowl until it holds soft peaks. Fold in the buttermilk, then add the bicarbonate of soda. Fold into the potato mix.
Heat a large non-stick frying pan over a medium heat, then add 1 tbsp butter and some of the oil. Drop 3-4 spoonfuls of the potato mixture into the pan, then gently cook for 3-5 mins on each side until golden and crusty. Keep warm on a plate in the oven while you cook the next batch, adding more butter and oil to the pan before you do so. You will get 16 crumpet-size boxty from the mix. Can be made the day ahead, drained on kitchen paper, then reheated in a low oven for 20 mins.
Heat the grill to medium and put the tomatoes in a heavy-based pan. Add a good knob of butter and a little oil, then fry for about 5 mins until softened. Grill the bacon, then pile onto a plate and keep warm. Stack up the boxty, bacon and egg, and serve the tomatoes on the side.
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