Preheat the oven to 180C/Gas 4.

Place the sweet potato on a baking tray and bake for 25-30 minutes. Allow to cool, then peel and mash. Set aside 50g of mash for the béarnaise and keep the rest for the Wellington.

Place the mushrooms gill-side up on a baking tray. Mix half of the miso paste with the oil and brush this over the mushrooms. Cook for 15 minutes then allow to cool for a few minutes.

Mix 150g of the sweet potato mash with the chopped spinach.

Season.

Break up the defrosted burgers and mix with the parsley, rosemary and onion jam or marmalade.

Season well.

Unroll the pastry sheet and spoon on the sweet potato and spinach mixture in a 6cm-wide strip along the centre, leaving a 3cm border at each end.
Place the mushrooms on top of the sweet potato. Lay the burger mix evenly along the top of the mushrooms.

Mix the remaining miso paste with the soya milk, and brush the edges of the pastry. Fold in the two short ends of pastry and bring the two long edges together so they overlap and seal everything in.

Turn the Wellington over so the seam is underneath and brush with the remaining soya-miso mix.

Make a few cuts across the top of the Wellington to let the steam out, then bake on a non-stick tray lined with greaseproof paper for 20 minutes, or until golden brown.

To make the béarnaise, blend the reserved 50g sweet potato mash with the vegan margarine, tofu and white wine vinegar. Add the tarragon and season with salt and lots of black pepper. Put into a serving bowl.

When the Wellington is cooked, remove it from the oven and allow to rest for 10 minutes.
Turn the oven up to 200C/Gas 6.

Place the asparagus on a baking tray, drizzle with olive oil, season with salt and pepper and roast for five to 10 minutes.

Slice the Wellington and serve with the asparagus, béarnaise and watercress dressed with a drizzle of extra virgin olive oil.