First poach the haddock in the milk. Heat the milk gently and once it starts to bubble, add the fish. Remove from the heat and allow to stand for 5 mins. Remove the fish and allow to cool. Remove the fish skin.
Melt the butter (or oil) over medium heat. Stir in the flour. Stir in the poaching milk. And cook gently, stirring till smooth. Add nutmeg, mustard and plenty of black pepper.
Meanwhile gently heat remaining oil and fry the leek till soft.
Stir in the cheddar and leek. Mix and remove from heat. Blanch spinach or chard and drain, and once cool, squeeze out excess liquid with your hands.
Peel the larger beetroot and slice into thin saucers. Put on a baking sheet, drizzle with half the oil and season with salt and pepper. Roast for 30 mins till soft.
Put a layer of beetroot in bottom of dish, spoon over cheese sauce, then add a layer of flaked haddock, and spinach.
Repeat, finishing with a layer of sauce and scattering of Parmesan and black pepper.