In a 12-inch skillet, heat 1/4 cup of the oil over medium-high heat. Add the potato, celery root, onion, turnip, rutabaga, garlic and thyme, and cook, stirring occasionally, until the vegetables are golden-brown on all sides, 7 to 8 minutes. Add 3/4 cup of the brisket cooking liquid. Cook on medium-high heat to reduce the liquid to a syrupy consistency, about 2 minutes. Add 2 Tbs. of the butter and stir until melted and well mixed with the vegetables. Remove from the heat and set aside.