Beetroot, potato and mushroom Wellington

1. Preheat the oven to 200°C, gas mark 6. In a roasting tin, toss the potatoes and beetroot with the oil; season, add a splash of water and roast for 30-35 minutes, until tender and golden.

2. Meanwhile, melt the butter in a large frying pan over a low-medium heat, then add the onion, garlic and lemon thyme; season and cook gently for 15-20 minutes, until tender. Scoop into a bowl, then return the pan to the hob. Turn the heat up and add the mushrooms plus a drizzle of oil; season. Fry, stirring, until most of the liquid has evaporated and the mushrooms are turning golden. Take off the heat and set aside. Boil the kettle. Put the spinach in a colander in the sink, then pour over the boiling water. Allow to cool, then use your hands to squeeze out as much liquid as you can and roughly chop.

3. Once all the elements are cool, stir the onion, roasted vegetables, spinach, feta and lemon zest into the mushroom pan. Stir through ¾  of the beaten eggs. On a large piece of baking parchment, roll the pastry into a rectangle roughly 30cm x 40cm. Spoon the filling down the middle, lengthways, leaving a little space at both ends. Fold one side over and brush the edge with a little of the remaining beaten egg before folding the other side over to form a join. Roll the Wellington over so that the join is on the bottom, then tuck the ends under so that they're sealed. 

4. Lift the Wellington (on the parchment) onto a large baking tray. Brush all over with the remaining egg, then use a sharp knife to score the top in a criss-cross pattern and cut 3 small air holes. Bake for 35-40 minutes, until golden. Leave to stand for 5 minutes, then slice and serve with steamed greens, if liked.