Ingredients

• 25g butter
• 2 onions, halved and thinly sliced
• 2 garlic cloves, finely chopped
• few thyme sprigs, leaves picked
• 1/2 small celeriac (about 300g)
• 2 medium potatoes (about 250g)
• 2 large parsnips (about 250g)
• 400-500ml vegetable stock
• Salt and pepper

Method

1. Preheat the oven to 190°C/375°F/gas mark 5. Melt the butter in a heavy- bottomed frying pan and add the onions and garlic. Sauté for about 10 minutes until soft but not coloured. Put in the thyme leaves, season and set aside. 
2. Meanwhile, peel the celeriac, potatoes and parsnips. Either use a knife or a
3. mandolin to cut into thin slices about 2-3mm thick, slicing the parsnips length-ways otherwise the rounds will be very small.
4. Neatly arrange the celeriac slices into the bottom of an ovenproof dish measuring approx 25x20cm. Season generously with salt and pepper, then sprinkle with half of the onion mixture. Layer the parsnips on top, then scatter with more salt and pepper, then the remaining onions and finish with a layer of potatoes.
5. Bring the stock to a simmer, then pour over the vegetables to just cover them.
6. Cover the dish with foil and bake for 45 minutes, then uncover and continue to bake for another 45 minutes or until the vegetables are cooked through.