Preheat the oven to 400 degrees F. Put the potatoes into a pan of boiling, salted water and cook for around 8 minutes, then drain and return to the pan. Chuff them up a bit by shaking the pan.
To prepare the meat, scatter a handful of finely chopped rosemary leaves over a large chopping board. Sprinkle over some salt and pepper and the ground fennel seeds. Roll the pork across the board, pressing down hard so all the flavorings stick to it.
Get a large roasting pan that your pork will fit snugly into, and place it on a burner over a medium-high heat. Pour in a little olive oil and place the pork in, fat side down, sprinkled with any flavorings remaining on the board. After a few minutes, when the pork fat is lightly golden, turn it over and add the garlic cloves, onion, celery and bay leaves to the pan. Place on the bottom shelf of your preheated oven for 1 hour, basting it halfway through. (For the last 20 minutes of cooking, you may need to cover the pork with a bit of damp waxed paper to stop it coloring too much.)
Get another roasting pan, into which you can fit the potatoes in 1 layer, and heat it on the stove. When hot, pour a glug of olive oil into it and add the butter, rosemary and garlic. Add the potatoes and toss them in all the flavors. Add the onions and all the balsamic vinegar and season with salt and pepper. Cook for 5 minutes on the burner to reduce the balsamic vinegar a little. Place the pan on the top shelf and cook for around 50 minutes, until the potatoes and onions are dark, sticky and crispy - removing the pan to toss the onions and potatoes halfway through.
After 1 hour, the meat should be cooked. Prick it with a sharp knife - if the juices run clear, it's done; if not, pop it back in the oven for another 10 to 15 minutes, keeping the potatoes warm. Remove it from the oven and let it rest on a plate for 10 minutes. Pour away most of the fat from the pan and mash up the garlic and onion. Place the pan over the burner and add the white wine. Simmer until the liquid has reduced by half, scraping all the meaty, marmitey goodness off the bottom to make a tasty little sauce, and season if necessary. Pass through a sieve into a serving pitcher. Then slice the pork and serve it with your incredible baked onions and potatoes, drizzled with the pan juices. This meal goes great with some nice greens or an arugula salad.
Recipe Excerpted from JAMIE AT HOME by Jamie Oliver. Copyright (c) 2008. Published in the U.S. by Hyperion. All Rights Reserved. Available wherever books are sold. www.jamieoliver.com