1.Heat half the butter and 2 teaspoons of the oil in a large frying pan; cook pork until browned all over. Add celeriac, cook, covered, for 10 minutes, turning occasionally, or until celeriac is tender.
2.Meanwhile, heat the remaining butter and oil in a medium frying pan; cook the onion, apple and mustard seeds, stirring, until the onion is soft. Add the stock; cook, uncovered stirring, for 3 minutes or until the apple is just tender. Remove from heat; stir in sage leaves.
3.Slice the pork thickly and serve with the celeriac and apple mixture.