Cook the pearl barley with the stock cube according to pack instructions. Reserve 4 tbsp of the cooking stock, then drain the grains well.
Bring a large pan of salted water to the boil, then add the parsnips, and cook until really soft. Drain well, tip back into the pan to steam dry for a few minutes, then roughly mash. Mix the ground linseeds with the reserved stock water, and leave to go gluey.
Put 2 tbsp of the oil in your largest frying pan with the onions and garlic. Fry gently until soft and golden, then stir in the mushrooms, rosemary and sage. Fry the mushrooms until they're golden too, and any liquid that comes out has evaporated. Scrape into a big mixing bowl and set aside to cool.
Grease a 22-24cm savarin or ring tin generously with oil. If it's not a non-stick tin, line it with thin strips of overlapping baking parchment.
Add the mashed parsnip, cooked pearl barley, gluey seed mix, grated vegan hard cheese (if using), chopped parsley and whole hazelnuts to the fried mushroom mixture. Season generously with salt and nutmeg, then mix everything together really well.
Spoon the filling into the tin, and press down firmly to fill the tin and flatten the top. Keep in the fridge for up to 24 hrs before baking.
Heat the oven to 200C/180C fan/gas 6 and cover the tin with foil. Bake for 45 mins, until a skewer poked into the centre of the mixture comes out piping hot.
Use a small palette knife or cutlery knife to release the filling all the way around, then sit a serving plate on top and flip over. Carefully lift off the tin, and top the wreath with some parsnip crisps if using and whole pumpkin seeds to serve – plus some extra rosemary sprigs if you like. Slice into wedges and enjoy.