For celeriac mash, bring milk, celeriac and potato to the boil in a saucepan over medium-high heat and cook until celeriac and potato are tender (15-20 minutes). Drain off milk (reserve), add butter to celeriac mixture and coarsely mash with a fork. Add enough reserved milk to form a soft mash consistency, season to taste and keep warm.
Meanwhile, preheat oven to 200C. Heat oil in a large frying pan over medium-high heat, add pork and cook, turning once, until well browned (1-2 minutes each side). Transfer to an oven tray and roast until just cooked through (2-3 minutes), then set aside to rest (5 minutes).
Return pan to the heat, add quince paste and wine and stir until smooth. Add stock, boil until reduced to a glaze, then add vinegar and thyme, and season to taste.
Serve cutlets with celeriac mash and frisée salad, with quince sauce spooned over cutlets.