Take the six big onions and slice a very small slither from the bottom so they can stand on their own. Take a sharp knife and cut a cross in the top to about three quarters of the way down. Keep all the skins own.
Put the onions in a large pot and cover with fresh cold water, then bring to the boil and then simmer for 15 minutes. Refresh them under cold water and set aside ready to be stuffed with the plant based haggis.
To make the haggis put the oats, pinhead oatmeal and nuts in a pot over a low heat and toast for about three to four minutes. Keep stirring the mixture to ensure it doesn't burn. Put the mixture in a bowl and set aside. Preheat the oven to 190C/375F.
In the empty pot add a couple of centimeters of water, add the onion, mushrooms and carrot then cook for about five minutes before adding the toasted oat mixture and all the other ingredients. Mix them all together ready for the mixture to be spooned into the large onions.
Remove the centers out of the onions leaving three to four outer layers. Set aside the centers and place the onions in a roasting tin. Spoon the haggis equally between the onions and bake for 40 minutes.
Serve with mustard mash, turnip and roasted lemon and garlic Brussel sprouts.