Heat oven to 180C/160C fan/gas 4. Season the pork and put in a large roasting tin.
Heat a large frying pan, add the oil and fry the lardons, shallots, celeriac and thyme until golden brown. Arrange around the pork with the garlic bulb.
Add the cider to the frying pan, bring to the boil, then pour around the pork. Cover with foil and roast for 1 hr. Remove the foil and cook, uncovered, for a further 45 mins.
Finally, add the peas and cook for 3 mins more. Leave the pork to stand, covered, for 15 mins before carving. Serve with buttery mash (add the soft roasted garlic cloves if you like) to soak up the delicious juices.