Boil the potatoes until tender. Drain, return to the pan and stand over a low heat for 1 minute to drive off any excess moisture. Add the milk and heat but don't boil. Remove from the heat, season with salt and pepper, then add the mustard and mash until smooth.
Meanwhile, put the cabbage in a pan of boiling water and cook for 5-10 minutes or until just tender. Drain well.
Stir the cabbage and spring onions into the potato. Add the cheese and keep warm.
Grill the gammon steaks under a preheated grill on medium heat for 10 minutes, turning occasionally. Grill the tomatoes for 3-4 minutes or until the soften slightly. Serve the gammon with the mash and grilled tomatoes.