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Bring a large pan of salted water to the boil. Put the leeks, broccoli and celery in a large steamer. Add the potatoes to the water and cook for 20 mins, with the vegetables steaming on top, until all are tender. Drain the potatoes, then mash with plenty of seasoning, 25g of the butter and all the yogurt.
While the veg cooks, pour the milk into a pan, add the flour, both mustards and remaining butter, and cook over a medium heat, whisking all the time, until smooth and thickened. Stir in half the cheese and season. Remove from heat.
Divide the steamed veg and peas between 8 individual pie dishes. Pour over the sauce and top with the mash, then sprinkle over the remaining cheese.
Pack into freezer bags and use within 3 months. To serve, unwrap and put the dishes on a baking tray in the cold oven, then set to 200C/180C fan/ gas 6. Bake for 50-55 mins until bubbling and hot all the way through.