For the pork belly, place the orange juice and balsamic vinegar in a sterilized, sealable, airtight container. Leave to infuse in the fridge for at least a week.
Preheat the oven to 160C/140C/Gas 3. Place the onions, galangal, chillies and bay leaves on the bottom of a heavy roasting tin. Rub the chopped garlic all over the pork belly and place the meat on top of the vegetables in the tin. Pour the sake into the tin, add 200ml/7fl oz cold water, cover with kitchen foil and roast for 45 minutes.
Remove the pork belly from the oven and reserve the stock. Press the pork belly by covering with kitchen foil and laying a heavy weight on top. Leave for 12 hours, or longer if you have the time.
For the pickled vegetables, pour the vinegar and 125ml/4fl oz of water into a saucepan. Add the rest of the ingredients to the pan, place over a low heat and cook gently, stirring until the sugar has dissolved. Set aside to cool.
Finely grate the cooled onion, radishes and mooli on a mandoline. Place in a bowl and stir in some of the pickling liquor.
Place 150ml/5fl oz of the orange-infused balsamic and the reserved stock in a saucepan and reduce down over a high heat. Whisk in the butter and simmer until it thickens.
Cut the pork belly into slices. Fry the pork in a hot frying pan or griddle, until golden on all sides. Coat the pork on both sides with the orange sauce.
Serve the pork belly slices with the pickled vegetables and leafy greens alongside.