Preheat the oven to 200°C / fan oven 180°C / Gas mark 6.

Melt 25g (1oz) of the butter in a large frying pan. Add the onions and cook over a low heat until softened. Add the apple slices and cook for another few minutes until tender. Add the sugar and cook gently until caramelised, then stir in the vinegar and thyme leaves.

Transfer the mixture to a shallow 20cm (8inch) baking tin. Level the surface and sprinkle with most of the grated cheese.

Melt the remaining butter. Cut the sheets of filo pastry in half, to give you 8 sheets that will fit over the baking tin. Brush these sheets lightly with melted butter and layer them over the apple and onion mixture. Tuck the pastry edges down the sides of the tin.

Bake in the oven for 12-15 minutes until golden brown. Cool for about 15 minutes, then invert the tart onto a large serving plate. Sprinkle with the remaining cheese and serve at once, garnished with sprigs of thyme.