Place the rhubarb, sugar and a splash of water in a pan and cook on a medium heat until the rhubarb is soft. Add the onions, garlic, tomatoes, thyme, vinegar, lemon juice and zest and the spices. Bring slowly to the boil, stirring occasionally, then simmer for about two hours, stirring frequently, until the chutney is thick.
Cut through the fat on the gammon steaks in several places with a pair of scissors. Place the olive oil in a pan on a medium heat and when the oil is hot, fry the gammon steaks for 3-4 minutes on each side.
Place the gammon and some of the chutney onto plates and serve with cooked green cabbage.