Cook the potatoes and mixed vegetables in a pan of boiling water for 15 to 20 minutes. When they're cooked right through, drain and put aside.
Heat a glug of olive oil and half the butter in a large frying pan and add the chestnuts. When they start to sizzle, add the potatoes and vegetables. Mash the vegetables up in the pan, then pat the mixture into a thick pancake shape. Fry on a medium heat for about 1/2 an hour, checking it every 5 minutes. When the bottom turns golden, flip it over bit by bit and mash it back into itself. Pat it out flat again and continue cooking until really crisp all over.
Preheat the oven and a roasting pan to 425 degrees F.
Unravel the pork sausage links and squeeze the filling between them until all 6 sausages are joined together. Do the same to your venison or beef sausages. Pat them to flatten them a bit. Drizzle with olive oil and massage this into your 2 long sausages. Sprinkle over a pinch of pepper, the rosemary and some nutmeg. Put 1sausage on top of the other and roll them up like a licorice wheel! Poke 2 skewers through, in a cross shape, to hold the sausages together.
Take the preheated roasting pan from the oven. Drizzle in some olive oil and add the onions. Season, add the remaining butter and stir. Place the sausage wheel on the onions and stick the bay leaves between the sausages. Drizzle with some more oil and roast in the preheated oven for around 40 minutes, or until golden and crisp. When your sausages and onions are done, your bubble and squeak should be ready too. If it still hasn't browned, put it under a hot broiler for 5 minutes.
Remove the sausages to a plate and place the pan with the onions on the stove top. Whack the heat up to full and stir in the flour, balsamic vinegar and stock. Bring to the boil and leave to thicken to a nice gravy consistency, stirring every now and then, and season to taste. Remove the skewers and cut the sausages into wedges. Serve the bubble and squeak with a good portion of sausage, a spoonful of onion gravy and perhaps some lovely dressed watercress.
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Recipe Excerpted from JAMIE AT HOME by Jamie Oliver. Copyright (c) 2008. Published in the U.S. by Hyperion. All Rights Reserved. www.jamieoliver.com