Haggis is a cooked product, it only requires heating up to serve. Traditionally the haggis is warmed gently in a pan of water. This method works very well, but if the casing splits - it means you will be serving Haggis soup!
We prefer to warm the Haggis in the oven, dry heat then even of the casing does split, the haggis will not be spoiled.
Haggis is usually served with mashed potatoes and mashed turnip, it is also common to sprinkle some whiskey over the haggis for a really traditional touch.