Dissolve salt and sugar in 100ml water over low heat, then stir in 400ml cold water, spices, herbs, and beer.
Submerge the pork in the marinade and chill in the fridge for 24 hours.
Preheat your oven to 200°C.
Remove the pork from the liquid and pat it dry with paper towels.
Brown the pork in 50g of foaming butter in a pan, then transfer to a tray and roast for 15 minutes.
Blend brioche, mustard powder, soaked seeds, and remaining butter into a seasoned paste.
Layer Swiss chard, potatoes, and onion in an ovenproof dish.
Prepare a roux with butter and flour, then whisk in milk until thickened.
Stir in stock and half the Gruyère until melted and season well.
Pour the sauce over the vegetables and top with the remaining cheese.
Reduce the oven temperature to 180°C.
Spread the mustard paste on the pork and bake it with the gratin for 45 minutes.
Rest the pork for 15 minutes while finishing the gratin if needed.
Carve the pork into six chops and serve with the gratin.