Start off by preheating your oven to 180 degrees Celsius. Peel and roughly chop the onion and garlic. Melt the butter in a pan and add in the onions and garlic. Reduce the heat and sweat in the butter for about 5 minutes.
Transfer the onions and garlic to a large bowl. Blitz the slices of bread in a food processor until they've turned to breadcrumbs and then place these into the bowl with the onion.
Pulse the nuts in the food processor until roughly chopped - about 10 seconds should be sufficient, before adding these into the large bowl as well.
Roughly chop the mushrooms and transfer to the bowl along with the grated cheese, salt, pepper, and thyme before cracking in the eggs and stirring well to mix.
If using the marmite or vegemite, heat the stock and then stir in the marmite until combined. Add the stock into the nut roast mixture and stir well again.
Grease and line a large loaf tin with baking paper and spoon the nut roast mixture into the tin, pressing down well into the corners and smoothing off the top.
Place into the preheated oven and bake for about 1 hour, until deep golden brown on top, and a skewer stuck into the loaf comes out clean.
Remove from the oven and allow to cool for 10 minutes in the tin before gently turning out and peeling off the baking paper.
Serve in thick slices and enjoy!