Preheat the oven to 350°. In a baking dish, toss the beets with the oil. Cover with foil and roast for about 1 hour, until the beets are tender when pierced. Let cool slightly, then peel the beets and cut them into small wedges.
In a large, deep skillet, heat 2 tablespoons of the goat butter. Add the celery root and thyme sprigs and season lightly with salt and pepper. Cook over moderate heat, stirring occasionally, until lightly browned in spots, about 5 minutes. Add 1/4 cup of the stock and simmer over moderate heat until nearly evaporated, about 2 minutes. Add the remaining stock, 1/4 cup at a time, and cook until the celery root is tender, 8 to 10 minutes total. Stir in the beets and cook until heated through, about 2 minutes. Discard the thyme sprigs. Swirl in the remaining 1 tablespoon of goat butter and season with salt and pepper. Serve right away.