Preheat your oven to 200°C/Fan 180°C/Gas 6. Trim and peel the red onions, then slice them into wedges. Trim and peel 200g of the carrots, halve them, then slice into 1cm-thick half moon. Slice the leaves off the cauliflower. Break the cauliflower into small florets, and chop the stem into 1cm-thick chunks. Trim and peel the beetroot. Chop them into 1cm-thick dice.

Scoop all the chopped veg into a roasting tin. Drizzle over 2 tbsp olive oil and season with salt and pepper. Toss the veg to coat in the oil and seasoning. Roast for 20 mins.

Pick the thyme leaves off half the woody sprigs (keep the rest for later). Peel and crush or grate the garlic. When the veg have roasted for 20 mins, take them out of the oven and add the thyme and garlic. Toss to mix. Roast for a further 20 mins till the veg are tender and lightly charred at the edges.

Meanwhile, rinse the baby leaf spinach and shake off any excess water. Tip it into a deep frying pan and set over a medium heat. Cook, stirring often, for 3-4 mins till the spinach has wilted. Tip into a sieve and press to squeeze out as much liquid as possible. Set aside to cool.

Take the veg out of the oven. Squeeze the spinach with your hands to squeeze out any remaining liquid (you can wrap it in a tea towel and wring it to really squeeze out the water). Tear it into the veg. Set aside to cool. If roasting the night before, let the veg cool then transfer to a tub and store in the fridge.

When you're ready to make the wellington, add the goat's cheese to the veg and lightly stir to mix together.

Take the pastry out of the fridge and let it soften at room temperature for 10 mins. If you're oven is off, preheat it again to 180°C/Fan 160°C/Gas 4. Dust your work surface with flour, if you have some. Take one block of pastry and roll it out to make a 28cm x 20cm rectangle. No flour? Roll the pastry out between 2 sheets of baking paper.

Transfer the pastry rectangle to a baking tray lined with baking paper. Spoon the roast veg down the middle of the pastry, leaving an 2cm gap around the edges. Roll out the second block of pastry to make a matching rectangle. Brush the edges of the pastry base with milk or water. Lay the second pastry rectangle over the top, pressing it to seal the edges. You can use a fork to crimp the edges together. Trim to neaten the edges up. Tranfer to the fridge to chill for 20 mins.

Brush the wellington all over with milk. Use a small, sharp knife to cut a few slashes in the top to let out any steam. Sprinkle over the sesame seeds. Bake in the oven for 40-45 mins till golden brown.

Meanwhile, fill and boil your kettle. Tip the vegetable stock powder into a heatproof jug trim and pour in 800ml hot water. Stir to dissolve. Set aside. Trim, peel and dice the onion. Trim and dice the celery. Trim, peel and dice the remaining 100g carrots.

Place a medium-sized pan on a medium heat. Add 1 tbsp oil and the diced veg. Fry the veg, stirring often, for 5-8 mins till the veg are browned. Add 1 tsp sugar, if you have some, and fry for an extra 2 mins till the veg are caramelised.

Add the cornflour. Cook and stir for 2 mins till the pan looks sandy. Trickle in the vegetable stock, stirring well between each addition, till the stock and veg are smoothly combined and there are no floury lumps. Add 1 tbsp tomato purée and the remaining thyme. Turn the heat up and bring to the boil, then turn down and pop a lid on the pan. Simmer for 10-15 mins till the gravy has thickened a little.

Pour the gravy through a sieve into a bowl to catch the veg. Press the veg to extract as much liquid as possible. Return the gravy to the pan and add 1 tbsp each balsamic vinegar and soy sauce. Gently warm through, then taste and add more balsamic, soy sauce, salt or pepper, if you think it needs it.

Let the wellington sit for 5 mins once it comes out of the oven, then transfer to a serving plate. Serve in slices with the gravy and the cranberry sauce.