Preheat oven to 350Ëš Fahrenheit. Boil potatoes, carrots, rutabaga and parsnips in well-salted water for about 15-20 minutes, or until potatoes are very tender. Drain vegetables and return to pan. Meanwhile, melt butter in a small skillet over medium heat until foam starts to subside. Add leeks and sauté until leeks are soft and starting to brown.
Mash the vegetables and then stir in the leeks (and butter), green peas and half the cheese. Stir in salt and pepper to taste then spread mixture in a casserole and top with the remaining cheese. Bake in the middle of the oven 40 minutes, or until cheese is starting to brown.
If there are only two in your house (like mine), you can put half of the Mash of Nine Sorts (before baking) in an oven-proof refrigerator container for another dinner. I just put it in the oven while the oven comes up to temperature and then set the timer for 40 minutes.