Preheat the oven to 180°C, 350°F, Gas 4.
Put the gammon into a roasting bag and then into a roasting tin and bake for 1½ hours, or the appropriate time according to the weight of your ham. (There should be a cooking guide on the packaging of the ham if you are unsure.)
Whilst the ham is cooking, prepare vegetables. Try and cut them similar sizes so that they all cook at the same rate. Leave them fairly chunky though so they don’t burn before the ham coating has fully cooked.
At this point the ham can be cooled and refrigerated until you are ready to glaze it and cook the prepared vegetables.
Carefully remove the ham from the roasting bag and put it on the oven tray. Arrange the parsnips, sweet potatoes, carrots and leeks around the sides of the ham, use another tray as well if you need to.
Carefully peel away the dark fat skin from the ham to reveal the white fat covering. Score this fat in a diamond pattern and spread generously with the wholegrain mustard and then gently sprinkle over the light soft brown sugar.
Return the ham and vegetables back to the oven and continue to cook for 30- 45 minutes, uncovered until the ham coating has crusted nicely and vegetables are tender and golden brown.
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