Nestle the roast back in with any juices from the plate. The liquid should come about halfway up the meat. Put the lid on and switch to braising heat: 12 coals in a ring underneath and 10 on the lid (about 325°F). You want a lazy simmer, not a rolling boil.
Braise for 1½ hours. Rotate the oven a quarter turn and the lid a quarter turn the other way every 15–20 minutes. Light a half chimney (about 20 briquettes) at the 45-minute mark and swap in fresh coals top and bottom as the old ones fade. If the liquid drops below a third of the way up the roast, add a splash of broth or cider.
Add the potatoes, carrots, and parsnips around the roast and push them down into the liquid. Flip the roast. Cover and braise for 1 hour more, keeping the coals fresh.
Tuck the apple quarters in on top of the vegetables. Cover and braise another 45 minutes to 1 hour.
Check for doneness. A fork should slide in and twist easily, and the meat should pull apart in thick pieces, about 195–205°F inside. The potatoes and carrots should be tender all the way through. If the roast still fights the fork, give it another 20–30 minutes.
Make the gravy and serve
Move the roast, vegetables, and apples to a platter and tent with foil. Remove the herb stems and bay leaves.
Take the coals off the lid and set the uncovered oven over about 15 fresh coals. Skim off extra fat and bring the liquid to a simmer. Stir the cornstarch into the cold water, then whisk it into the pot. Simmer for 3–5 minutes, until the gravy coats a spoon. Taste and season with salt and pepper.
Pull the roast into big pieces, return it to the pot with the vegetables and apples, and ladle the gravy over the top. Finish with parsley and serve straight from the cast iron.