Preheat the oven to 200C/400F/Gas 6.
Heat a large saucepan over a medium heat. Add the oil and, once hot, add the onion and a pinch of salt. Cook the onions for 4-5 minutes, or until softened. Add the carrots, swede and parsnip, plus another pinch of salt. Place a lid on and gently fry for 10 minutes, or until softened a little.
Remove the lid, sprinkle in the flour and stir around to coat the vegetables and soak up any cooking juices. Add the stock, pearl barley and thyme and stir to mix well. Simmer for 15-20 minutes to thicken. Add salt and pepper to taste.
Meanwhile, heat a large pan of water and cook the sliced potato in boiling water for 1-2 minutes to soften slightly. Drain and then put them into cold water to cool them and make them easier to handle.
Pour the vegetable mix into a large ovenproof dish about 21x29cm/8x12in. Arrange the potato slices over the top of the vegetables, seasoning between layers with salt and pepper to taste and finishing with a neat layer of slices. Dot over the butter and finish with a pinch of salt and pepper. Place into the preheated oven and cook for 45-50 minutes, or until the filling is bubbling and the potatoes are golden-brown on top.