Beet & carrot Wellington

Preheat the oven to 180°C, gas mark 4 and bring a kettle of water to the boil. Put the lentils, porcini and bicarbonate of soda in a large roasting tin and season. Pour over 600ml just-boiled water, then nestle in the beetroot and carrot chunks. Cover tightly with foil and poke a few small holes in the top with a skewer. Roast for 45 minutes, then uncover and increase the oven temperature to 220°C, gas mark 7. Scatter over the chestnut mushrooms and drizzle over the oil. Add a splash more water if the lentils are looking dry. Roast, uncovered, for 15-20 minutes until the mushrooms are golden and the lentils and vegetables are tender. Set aside to cool.

Mix the breadcrumbs with 3 tbsp of the milk alternative and set aside. Use tongs to lift the root veg out of the roasting tin; set aside on a plate. Put ½ the mushroom and lentil mixture in a food processor and pulse until smooth. Stir into the remaining lentils with the root veg, tarragon, 1 tbsp mustard, the Marmite, garlic, vinegar and breadcrumb mixture, then season.

Roll the pastry out on a sheet of baking parchment to a roughly 35cm square. Shape the filling into a log in the middle of the pastry (about 24cm x 15cm). Beat the remaining 1 tbsp mustard with the remaining 2 tbsp milk alternative and brush over the exposed pastry. Bring up the sides of pastry on either side of the length of the filling so they overlap on top, pressing firmly to seal. Fold up the ends and seal, trimming any excess if needed. Roll the Wellington over, then lift it onto a large baking tray, seam-side down. Brush all over with the remaining mustard glaze and score for decoration. Chill for at least 20 minutes (up to 24 hours).

Preheat the oven to 220°C, gas mark 7. Bake the Wellington for 30 minutes, checking halfway and covering with foil if browning too much. Turn the oven temperature down to 180°C, gas mark 4, rotate the Wellington and bake for 30-40 minutes until golden brown and piping hot in the centre. Leave to stand for at least 5 minutes before slicing. Serve with vegan gravy and greens, if liked.