Chop potatoes, parsnips and celery root into 1/2 inch pieces. Boil until tender and mash.

Meanwhile, cut leeks in half lengthwise and rinse under cold water to remove any soil, then slice into thin half rounds.

In a sauté pan over medium low heat, melt the butter and then add the leeks and garlic. Cook until the leeks begin to caramelize and turn brown, being carful not to burn the garlic.

In a casserole dish, combine the vegetable mixture, leeks, cream, and Brie. Bake uncovered at 425 degrees F for 20 - 30 minutes until heated through and browned on top.