For the pork, pour 100ml/3½fl oz cold water into a large saucepan. Add the sea salt flakes and sugar and heat over a low heat until dissolved, stirring well. Remove the pan from the heat and add 400ml/14fl oz cold water, the peppercorns, garlic, bay leaves, unsoaked yellow mustard seeds, thyme, parsley and beer.
If the pork loin rack fits into the pan, add it to the pan. Otherwise, place the pork loin rack in a large bowl and pour over the liquid. Cover and chill in the fridge for 24 hours to marinate, turning the pork occasionally.
When the pork has marinated, preheat the oven to 200C/180C Fan/Gas 6.
Remove the pork from the marinade and pat dry with kitchen paper.
Heat 50g/1¾oz of the butter in a large frying pan over a medium heat. When the butter is foaming, add the pork loin rack and fry for a few seconds on all sides, or until lightly browned all over. Transfer the pork loin rack to a roasting tray and roast in the oven for 15 minutes.
Meanwhile, blend the brioche, English mustard powder and soaked yellow and brown mustard seeds to a paste in a food processor with the remaining butter. Season, to taste, with salt and freshly ground black pepper, then set aside.
For the gratin, arrange half of the Swiss chard in a layer at the bottom of an ovenproof dish and top with half the potatoes. Sprinkle over the onion, then top with layers of the remaining Swiss chard and potatoes.
Melt the butter in a saucepan over a medium heat. Add the flour and whisk well to combine with the melted butter. Continue to cook for 1-2 minutes, whisking regularly, then gradually pour in the milk, whisking continuously, until all of the milk has been incorporated and the mixture has thickened to a smooth sauce.
Whisk in the stock and half of the Gruyère and stir until melted. Season, to taste, with salt and freshly ground black pepper.
Pour the sauce all over the chard and potatoes. Sprinkle over the remaining Gruyère.
After the pork has been cooking for 15 minutes, remove it from the oven and decrease the temperature to 180C/160C Fan/Gas 4.
Spread the brioche and mustard paste all over the top of the pork loin rack in a thick layer, then return the pork to the oven, along with the Swiss-chard gratin. Cook both for a further 45 minutes, or until the pork is cooked to your liking and the crumb has crisped up and turned golden-brown, and the Swiss-chard gratin is bubbling and golden-brown.
When the pork is cooked to your liking, set it aside to rest for 15 minutes (keep warm). Continue cooking the Swiss-chard gratin for a further 15 minutes, if necessary; otherwise, keep warm.
To serve, carve the pork loin rack into six chops. Place one onto each serving plate and spoon the Swiss-chard gratin alongside.