Method
1. Fill and boil your kettle. Scrub the Jersey Royals and chop them into bite-sized pieces. Snap the woody ends off the asparagus, then slice the tender tips into 3cm-long pieces. Trim the dry end off the broccoli stalk, then slice the stalk into thin rounds. Chop the florettes into bite-sized pieces.
2. Pour the hot water from the kettle into a large pan and add a pinch of salt. Add the potatoes. Boil for 8-10 mins till just tender. Add the broccoli and boil for a further 3 mins till bright green. Finally, add the asparagus pieces and boil for 2 mins till just cooked. Drain the veg and set aside.
3. Trim the roots and woody greens off the leeks, then rinse out any grit. Thinly slice the leeks. Pick the thyme leaves off their woody sprigs.
4. Set a large, clean pan over a medium heat and add 1 tbsp olive oil. Add the leeks and thyme leaves with a pinch of salt and pepper. Gently fry, stirring often, for 10 mins till glossy and fragrant.
5. When the leeks are soft, sprinkle in the cornflour and stir to combine. Cook for 1 min, then gradually whisk in the milk a little at a time. Crumble in the vegetable stock cube and add 150ml water. Bring the mixture to a boil, stirring constantly. Lower the heat and gently simmer for 5 mins till the sauce has thickened.
6. Meanwhile, coarsely grate the Cheddar. Finely grate the zest from the lemon. Add these to the sauce and squeeze in the juice from half the lemon. Tip in the cooked vegetables and stir well to combine. Taste and add more salt, pepper or lemon juice, if you think it needs it.
7. Preheat your oven to 180°C/Fan 160°C/Gas 4. Dust your work surface with flour, if you have some. Unwrap the pastry and roll it out to make a large sheet the same shape as your pie dish, approximately ½cm thick. No flour? You can roll the pastry out between 2 sheets of baking paper.
8. Spoon the mixture into a 1 ltr pie or baking dish. Lay the pastry over the top, lightly crimping it to the edges of your dish with a fork.
9. Using a sharp knife, cut a small hole in the centre of the pastry to help any steam escape (this will help the pastry cook evenly). Brush the pie with milk or beaten egg, if you have some. Place the pie onto a baking tray and bake for 25 mins till the pastry is golden and crisp. Serve with your favourite vegetable side dishes.