Start by prepping all the vegetables, wash and cut potatoes into roughly 1inch pieces (do not peel). Peel and cut celeriac, swede & parsnip into 1inch pieces. Place all the prepped root veg together in a bowl and cover with water to stop them browning.
Slice the leeks into 1 cm rounds, de-skin the garlic cloves and crush gently with the back of your knife. Place in a frying pan on a medium heat with 25g butter, a generous pinch of sea salt, freshly ground black pepper & thyme stalks. Allow the leeks to golden then turn down the heat until they have softened, about 10mins. Once softened, remove from the heat, discard the thyme stalks and put to one side for later.
Now to part-boil the root veg, bring a large pan of salted water to the boil, place all your prepped roots in the pan and cook for 5 minutes. Then drain and leave to cool in a colander.
Whilst the veg is cooling, you can make the bechamel. Melt the 50g butter in a medium saucepan, add 50g plain flour and stir with a wooden spoon. You want to cook the flour, so continue stirring for 2 minutes on a low heat. Then add roughly 100ml of the milk to the pan, and whisk, keep adding the milk in 100ml pours until it’s all gone, whisking vigorously in-between. Don’t panic if initially it seems to clump, once you’ve added more milk it will start to turn into a smooth sauce. Bring the bechamel up to a simmer whilst continually whisking for 1-2minutes, once thickened remove from the heat.
Add 100g grated Cheddar, 100g grated Red Leicester, 2 tspn dijon mustard, freshly ground black pepper to the bechamel and combine whilst still hot. The cheese will melt in the residual heat from cooking. Taste to check seasoning, add more mustard or black pepper to suit your palate.
Now it’s time to fill the pie dish! If you are baking the pie straight away then it’s ok if the elements are still a little warm, if you’re prepping the day ahead or making them to freeze, wait until everything is room temp before assembly. Like you would a lasagna, you want to layer the fillings, start with a couple of generous spoonfuls of bechamel, then the leeks, then the root veg, finishing with a layer of sauce and then the 50g reserved grated cheese.
Remove your puff pastry from the fridge and cut a pie lid to match the size of your dish, push down the edges of the pastry at the rim to seal the filling inside. Then brush with beaten egg and decorate with any excess pastry. We chose the word PIE but you can creative! Also a fun task for any children to get involved.
Bake at 180 Fan / Gas mark 4 for 35-50 minutes depending on the pie size or until the pastry is deep golden brown and the filling is starting to bubble out of the edges. Serve with lots of green veg & a dollop of mustard.