1) Peel and slice the turnips thinly. Peel and dice the parsnips (cutting the vegetables small and thin will help speed up cooking time).
2) Add the sliced and diced vegetables to a large saucepan and cover them with cold water. Add salt to the water and bring to a boil over medium heat. Boil the vegetables for about 8 to 10 mins until quite soft. Vegetables are cooked when you can easily insert the tip of a knife.
3) Drain the vegetables and place them back into the saucepan. Add the butter and thyme. With a potato masher, mash all ingredients together. Season with salt and pepper, to taste. Serve warm.