Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and chop the potatoes into equal-sized chunks. Pop into a pan and cover with boiling water. Add a pinch of salt. Cover. Bring back up to the boil. Simmer for 12-15 mins till soft.
Trim the leeks. Rinse out any grit and finely slice them. Finely grate the zest from the lemon. Put the leeks and zest in a deep frying pan or wok. Add the smoked haddock fillets and 1 bay leaf. Cover with 300ml milk.
Put the pan on a medium heat. Bring to the boil, then turn the heat down and simmer for 5 mins. Turn off the heat. Lift the haddock out of the pan and pop on a plate to cool. Take out the bay leaf and discard. Keep the milk in the pan for later.
Coarsely grate the Cheddar. Finely shred the spring greens. Place the pan with the milk back on the heat. Swirl in 2 tsp Dijon mustard. Drop in the spring greens and a third of the Cheddar. Gently cook for 5 mins till the spring greens are tender. Take off the heat.
Trim and finely slice the salad onions. The potatoes should be cooked by now. Drain them. Return to the pan. Add plenty of salt and pepper and mash till smooth. Add the salad onions and stir to mix. This is your champ.
Flake the haddock fillets and gently stir them into spring greens and Cheddar sauce.
Spoon the haddock and sauce into a small baking dish. Cover with the champ. Sprinkle over the remaining Cheddar. Scatter over the breadcrumbs.
Bake the pie for 30 mins till the top is golden and crispy. Serve with wedges of the zested lemon.