Photo: Dan Jones
Make sure both the potatoes and the greens are very dry, to ensure that the bubble and squeak cakes become crispy. If the potatoes aren't already mashed, mash them with the back of a fork into a coarse purée. If using greens, chop them into small pieces.
Heat ½ tablespoon of the olive oil in a small frying pan. Add the sliced leek and cook for 3 minutes, stirring until lightly browned. Mix the cooked leek, mashed potato, greens and seasoning in a bowl. Shape the mixture into 4 discs, about 2.5cm thick; chill for 1 hour.
Next, make the sauce. Put the vegetable or chicken stock, wine, double cream and the 25g butter into a medium-sized saucepan, bring to the boil and bubble rapidly for 7-8 minutes until it has reduced by about three-quarters and is the consistency of single cream. Mix the teaspoon of butter with the flour into a smooth paste. Whisk into the sauce and simmer for 1-2 minutes. Keep warm, but don't overheat it or the sauce will split.
Heat the remaining oil and knob of butter in a large, nonstick frying pan. Fry the bubble and squeak cakes over a medium heat for 4-5 minutes on each side, until crisp and really golden.
Meanwhile, poach the eggs (one or two per person) in a large pan of gently simmering salted water (add a teaspoon of white wine vinegar) for about 3 minutes, then lift them out with a slotted spoon and drain briefly on kitchen paper.
Reheat the sauce, whisk in the wholegrain mustard and season to taste; serve with the bubble and squeak cakes and the poached eggs.