Preheat oven to 325°F (165°C). Heat olive oil over medium heat in a heavy pot or any ovenproof deep dish with a cover. Add bacon and cook uncovered until crisp. Transfer bacon to a bowl. Add butter to bacon fat in pot and cook pearl onions until browned on the outside. Using a slotted spoon, transfer browned onions to a bowl. Add pork to pot and brown over high heat.
Remove pot from heat once pork has browned and add back onions. Pour Calvados over meat and, using a long lighter or match, light the calvados and let the flame burn down and the alcohol burn off.
Put pot back over medium heat and add chopped apple, mustard, celery, thyme, sage, apple cider and stock. Season with ground pepper and bring to a gentle simmer. Cover and put in oven for 2 hours.
After 2 hours, skim any excess fat from the top and stir in slurry (cornstarch - water mixture) and the cream. Put lid back on and return to oven for another 15 minutes.
To serve, remove casserole from oven, stir well and either transfer to a big serving bowl or leave in casserole.