Place the marinade in a small pan and cook until one third of the liquid remains. Pre-heat the oven to 350 Fahrenheit (175 Celsius).
Peel the root vegetables and cut them into thin slices.
Grease a fireproof dish with butter. Arrange the root vegetables in layers, creating a roof tile pattern. Pour over the reduced marinade and spread the butter on top. Sprinkle with the thyme and bake in the oven until the root vegetables are tender and start to acquire golden edges, about 50 minutes.