microwave is easier, break the haggis down into smaller pieces/chunks then cover and cook. or cut haggis into slices and cook in oven crispier that way.
I have always boiled then simmered the Haggis wrapped in foil. Beware when cutting through the plastic covering. Very Hot liquid is likely to squirt out. Painful.
I've frozen the leftovers...though I eat them cold also.
Fried egg as said, or throw an egg or 2 in the same pan and make a scramble with shallots or onions.