Heat your oven to 180°C/Fan 160°C/Gas 4. Peel and chop the potatoes into equal-sized chunks. Pop in a pan and cover with boiling water. Add a pinch of salt. Cover. Bring back up to the boil. Simmer for 7 mins. Carefully lower in the eggs and cook them with the potatoes for 6 mins. The potatoes should be soft after this time. If not, remove the eggs and cook the potatoes till they're tender when pierced with a knife.
While the potatoes simmer, trim the leeks. Halve them, 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, pop on a lid and simmer for 5 mins. Turn off the heat. Lift the haddock out of the pan and pop on a plate to cool. Fish the bay leaf out and discard it. Keep the poaching 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 eggs and potatoes should be cooked by now. Lift out the eggs and plunge them into ice-cold water. Drain the potatoes. 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.
When the eggs are cool enough to handle, peel them and cut them into quarters. Flake the haddock fillets and gently stir them into the spring green and cheddar sauce.
Spoon the haddock and sauce into a small baking dish. Push in the soft boiled egg quarters. Cover with the champ. Sprinkle over the remaining cheddar. Scatter over the breadcrumbs.
Bake the fish pie for 30 mins till the top is golden and crispy. Serve straight away.