Peel and thickly slice 2 onions and roughly chop the bacon. Drizzle 1 tablespoon of olive oil into a large non-stick frying pan, place on a medium heat, then add the bacon and cook, stirring, for 3 minutes. Add the sliced onions and a pinch of sea salt and black pepper, and cook for 5 minutes, then leave to cool.
Meanwhile, use a sharp knife to carefully debone the thigh meat, placing the bones in a 40cm x 30cm roasting tray. Speed-peel and add the carrots (save the peelings), and peel and quarter the remaining onion. Chuck the carrots, their peelings and the onions all in the tray with 1 teaspoon of oil, bringing the carrots and bones to the top.
Scrub the potatoes, prick them a few times with a fork and microwave on a high heat (800W) for 15 minutes, or until starting to soften.
Preheat the oven to 180°C/350°F/gas 4. Sprinkle a little salt and most of the sage over the chicken, cover with greaseproof paper, then bash with a rolling pin to flatten to 1.5cm thick. Remove the paper then dollop over the cranberry sauce. Spread 1 teaspoon of mustard over each slice of bread, cut in half and lay each piece over each thigh, then sprinkle over the bacon and onions.
Roll up the thighs to seal the filling inside, then tie with string or secure with cocktail sticks and transfer to a second 40cm x 30cm roasting tin. Gently squash the potatoes with a potato masher and nestle them into the gaps. Drizzle over 2 tablespoons of oil, sprinkle with the remaining sage and a pinch of salt and pepper.
Roast on the middle shelf of the oven for 1 hour, or until the potatoes are golden and the chicken is cooked through, basting halfway. When there’s 40 minutes to go, place the bone tray on the bottom shelf to roast alongside.
Meanwhile, trim and halve the cabbage lengthways, then slice each half into three wedges. Place them on top of the carrots and roast for 20 minutes, or until charred. Put the roasted cabbage and 1 carrot aside for later, then tip the remaining carrots into the chicken tray to continue roasting.
Boil the kettle. Place the bone tray over a medium-low heat. Mash the carrot into the juices with a potato masher, then stir in the flour. Gradually pour in 750ml of boiling water, stirring and scraping up any gnarly bits from the bottom. Season to taste, then let it bubble to your desired consistency, stirring regularly. Strain through a sieve into a jug.
When you’re almost ready to serve, scatter the frozen peas into the frying pan in a single layer, drizzle with 1 teaspoon of oil and arrange the roasted cabbage on top. Cover and place on a high heat to steam for 8 minutes, or until piping hot.
Remove the string from the chicken, slice each thigh into 3, take everything to the table and serve.
