Heat oven to 350 (180C). Place the tahini paste and vegetable stock in the base of the dutch oven over a high heat on the stove top. Whisk together and when it comes to the boil add the swede, parsnip and sweet potato. Season with salt and white pepper, add the chopped thyme. Place the lid on the dutch oven and put it in the heated oven for 40 minutes.
Towards the end of the cooking time melt the butter in a small saucepan over a low to medium heat. When it has melted stir in the flour and cook for a few minutes until a paste has formed.
Take the stew out of the oven and stir in the small broccoli florets. Add the cooked butter and flour mixture and stir constantly but gently for a few minutes with a wooden spoon until the sauce thickens and becomes smooth. Let the stew stand for 5 minutes before serving to allow the broccoli to cook.