1. In a pot, boil potatoes and celeriac in plenty of salted water for 20 minutes, or until soft. Drain and return to pan. Mash with 2 tablespoons margarine and olive oil. Add water, as needed, to reach desired consistency. Season with salt and pepper and add more olive oil if a richer mash is desired.
2. In another pot, bring enough water to cover cabbage to a boil. Add cabbage and blanch for a few minutes, then rinse under cold water and drain.
3. Stir the blanched cabbage into the mash, cover, and set aside.
4. In a pan, heat the additional margarine. Add the leek and fry until softened.
5. Stir leek into mash and serve.
This is my take on an old Scottish/Irish recipe. Adding celeriac to the mash infuses the dish with it's subtle, celery-like taste. Another option is to form the finished Colcannon into large, round patties and fry these in hot oil, turning frequently until they are brown and crispy on the outside.