Trim the turkey breast so that it's a more oval shape (which means cutting pieces off the neck end, that you will use later). Tie with a 2cm (1inch) gap between each loop of string, to give a rounded shape.
Heat 2-3 Tbsp cooking oil in a large skillet. Season turkey and fry in pan until golden all over. Remove from pan and cool. Place in refrigerator to firm up before removing the string.
For the stuffing, cook onions in 2 Tbsp butter for a few minutes until soft. Remove from heat and cool. Roughly chop turkey trimmings and blitz in food processor until smooth. Add sausages and blitz again 30-40 seconds until mixed together. Season and mix in eggs (with wooden spoon or blitz again).
Spoon stuffing into a large bowl. Mix in onions, cranberries, chestnut puree, and breadcrumbs. Add seasoning. Chill in fridge.
Thin out bacon by rolling between two sheets of cling film (you'll need to do this in batches). Chill bacon rashers on tray until needed.
Roll puff pastry on lightly floured surface to a rectangle approximately 45X35cm (18X14inches). Lay bacon rashers on top of pastry, leaving a 3-4 cm (1 1/4 inch) border all the way around. Slightly overlap the bacon rashers as they are laid on the pastry to allow for shrinkage as it cooks. (You'll need about 14-15 rashers).
Spread cranberry stuffing over bacon, reserving a few tablespoons. Remove the string from the turkey and lay the breast lengthwise, presentation side down, on top of stuffing. Spread reserved stuffing along centre and top of turkey. Rap turkey and stuffing roll with bacon by lifting 4-5 bacon rashers at a time and pressing into the turkey breast. Continue until both sides have been lifted. Lay remaining rashers lengthwise on the top, covering stuffing that was spread there.
Bring one long side of pastry up and over the turkey. Brush the other long side with beaten egg along the 3-4cm (1-1/4inch) border, and press to seal. Brush both ends with beaten egg and fold up to seal. Turn carefully so that wellington is presentation-side-up and place on lightly oiled baking tray. Chill.
To cook wellington, preheat oven to 200°C, Gas mark 6. Bake 1 hour, 15minutes to 1 hour 25 minutes, max. After first 45 minutes, brush wellington with beaten egg to colour the pastry. Once cooked, remove from oven and rest for 15-20 minutes before lifting carefully on to a carving board with two large fish slices.
While wellington is baking, make the sauce. Melt 2 Tbsp butter in a pan and add shallots or onions. Cook on medium high heat until golden. Add honey and cook until almost caramelised. Add red wine vinegar and red wine. bring to the boil and reduce by half. Add water or stock and return to simmer. Mix flour with 1 Tbsp butter. Whisk into sauce a bit at a time. Return sauce to boil and simmer a few more minutes. Strain through a sieve and season. Slice the wellington and serve with potatoes and brussel sprouts.