Three-root boulangère - River Cottage

Ingredients

• 30g butter or dash of oil
• 2 onions, halved and thinly sliced
• 2 garlic cloves, sliced
• 1 small celeriac
• 2 large potatoes
• 3 large parsnips
• A couple of sprigs of thyme, leaves only, chopped
• 3 sage leaves, finely chopped
• About 1.2 litres vegetable stock
• Sea salt and freshly ground black pepper