Heat the butter and oil in a large deep sided pan with a lid and gently fry the onions very slowly until deep golden and sweet - about 25 minutes - stirring every now and then. Add the garlic and cook for a further minute, then add the potatoes. Sprinkle over the thyme and seasoning and cover the pan with a lid. Leave the potatoes to cook for about 10 minutes, until softened, then remove the lid and carefully move them around using a spatula to help them to brown evenly and coat in the onions and garlic. Replace the lid and cook for a further 5-10 minutes until completely tender. Scatter the cheese liberally over the potatoes and press down with the back of the spatula to help the base crisp a little. Cover again and leave until the cheese has melted. Depending on your dexterity, you may wish to turn the truffade out onto a plate before serving or just use a large serving spoon.
