Preheat oven to 220c.
Pat the pork dry and season with salt and pepper. Heat oil in a large oven-proof pot or Dutch oven with a lid. Brown pork on all sides, turning with tongs, 6-8 minutes per side.
Transfer pork to plate. Pour off excess fat from pot. Add onion and 1 teaspoon salt. Sauté over medium heat, stirring occasionally, until onions are very soft ad deep golden brown, 18-20 minutes.
Add brandy and stir to deglaze pan.
Add apple, garlic and thyme. Cook, stirring, 30 seconds.
Return pork to pot, nestling it down in the onions. Add cider and chicken stock.
Cover pot and place in oven. Reduce heat to 160c. Braise until meat is very tender, 2 1/2-3 hours. Return pot to cooktop. Transfer pork to a cutting board.
Boil onion and apples until thickened and liquid slightly reduced, 1-2 minutes. Stir in mustard. Season to taste with salt and pepper.
Cut pork in serving pieces and arrange on serving platter or individual dinner plates.
Spoon onion apple confit over and around the meat.
Serve with a lightly dressed rocket or watercress salad.