Heat the oil in a large, heavy-based frying pan and cook the gammon steaks over a high heat for 5 minutes, turning once. Remove with a slotted spoon and keep warm.
Melt the butter in the pan, add the onions and thyme leaves and cook over a low heat, stirring occasionally, for 15 minutes until softened and beginning to caramelize. Stir in the marmalade, mustard and stock and bring to the boil, then gently simmer for 2–3 minutes until beginning to thicken.
Return the warm steaks to the pan and simmer for a further 3 minutes until the sauce is thick and sticky and the steaks are piping hot. Serve with instant mash, if liked.