Preheat oven to 175C fan.
Bring a large cast iron pot or braising/shallow casserole to medium/high heat with 2 tablespoon olive oil. Add the onion, carrot and celery with a big pinch of salt and sauté for 5 - 7 minutes until well browned.
Add the garlic, mustard, brown sugar, fennel seeds, thyme, sage bay leaf with another pinch of salt and a good crack of pepper and sauté for a further 2 - 3 minutes.
Pour in the beer and use a flat wooden spatula or spoon to deglaze the bottom of the pot, scraping off any fond to mix into the braising liquid.
Score the pork shoulder skin, drizzle with a light layer of olive oil and season very generously with a layer of salt (about 1 - 2 tablespoon worth). Gently place the pork into the pot with the skin side up.
Slowly fill the pot with chicken stock, until the liquid sits just below the skin (don't pour the stock over the pork, the skin needs to stay dry, just pour it into the side of the pot). You want as much skin exposed to air as possible. You may not use all of the chicken stock but leave aside as you may need it later.
Transfer to the oven and cook for about 4 hours. Check every hour - we want most of the pork to stay submerged in liquid (excluding the skin) so top up with chicken stock as needed.
After 3 ½ - 4 hours, the skin should have completely hardened and turned into crispy crackling. If not, crank the temp up to 220 for 15 - 20 minutes.
After about 4 - 4 ½ hours total in the oven, take the whole dish out of the oven, transfer the pork to a seperate tray to cool for 15 - 20 minutes.
If making gravy, strain the braising liquid (keep the carrots and garlic for serving) then remove as much fat as possible from the liquid (a thin layer of fat on top is fine but remove as much as you can).
To make the gravy, melt the butter in a small saucepan over medium/low heat then add the flour. Whisk and allow the flour to get a little toasty for 1 - 2 minutes. Add about ¼ cup of the braising liquid and whisk until smooth. Continue adding braising liquid and whisking until you have a smooth gravy. If its too thick, add some extra chicken stock or water until desired thickness. Season with a pinch of salt and pepper if needed.
Cut up pork, serve with potatoes, peas, carrots from the braise and the garlic. Pour over gravy and enjoy!