Preheat oven to 400˚F and line a 1lb loaf pan with a long strip of nonstick paper; use some of the butter to grease the tin and paper well.
Melt most of the remaining butter in a medium sized saucepan, add the onion and fry for about 10 minutes until tender but not browned. Remove from the heat.
Grind the cashew nuts in a food processor with the bread and garlic, and add to the onion, together with the water or stock, salt, pepper, grated nutmeg and lemon juice to taste.
Mix all the stuffing ingredients together.
Put half the cashew nut mixture into the prepared pan, top with the stuffing, then spoon the rest of the nut mixture on top.
Dot with the remaining butter. Stand the pan in another pan to catch any butter which may ooze out, and bake for about 30 minutes, or until firm and lightly browned. (Cover the roast with foil if it gets too brown before then.)
Cool for a minute or two in the tin, then slip a knife around the sides, turn out the nut roast and strip off the paper.
Decorate with sprigs of parsley and small slices of lemon, and surround with roast potatoes, if you're serving them.