To make the duxelles
Place the chopped mushrooms in a food processor and pulse until they are about the size of grains of rice.
Heat the vegetable oil in a large saucepan on a medium heat and cook the chopped shallot until translucent. Stir frequently, taking care not to let it catch. This will take about 20 minutes.
Add the chopped garlic and the mushrooms to the pan and cook uncovered on a medium heat for about 30 minutes. The mushrooms will release all of their moisture, which will be cooked off, intensifying the flavour. Once the mushrooms have cooked and darkened in colour, add the vinegar and cook for a further 10 minutes.
Coarsely blend the olives, capers and the olive oil in a food processor. Add to the mushroom mix and remove from the heat. Season to taste. Chill until ready to use.
To make the rough puff pastry
Sift the flour and ½ teaspoon salt into a bowl. Add the diced butter and 140 ml cold water. Mix together lightly by hand. Don’t knead it too much, as this will make your pastry tough.
Turn the pastry out onto a well-floured surface, dust with more flour, then roll out to a rectangle about 1 cm thick, dusting any butter that starts poking through with more flour.
Fold into three widthways, as though folding a letter, then roll out again to about 1 cm thick, then repeat the folding for a final time. Roll the pastry into a rectangle approximately 45 cm x 30 cm. Cover and chill until required.
To prepare the cavolo nero lining, separate the leaves and wash well. Strip out the main veins with a sharp knife, keeping the leaves as whole as possible. (You can cook the veins separately and serve as a side, if you wish.)
Blanch the leaves in boiling salted water for about 5 minutes, then drain and refresh in ice-cold water. Drain well and lay flat on a clean tea towel to get rid of all the last drops of water.
Heat the oven to 170°C/325°F/gas mark 3. If you’re not making your own pastry, roll your shop-bought pastry into a rectangle approximately 45 cm x 30 cm.
Arrange the cavolo nero leaves on top of the pastry, leaving a 3 cm gap all the way around the edges. Spread the mushroom duxelles on top of the cavolo nero.
Place the vegan roast (uncooked but defrosted) on top of the duxelles. Fold the long sides of pastry over the roast first and then fold up the ends to create your wellington. Press all openings with a fork to ensure the parcel is fully enclosed.
Place on a baking tray lined with greaseproof paper. Wash with soy milk. Score with a sharp knife and bake for 50-55 minutes until golden and piping hot.