1.Preheat oven to 200°C. Oil two 12-hole (2-tbsp/40ml) deep flat-based patty pans.
2.In a large frying pan, heat oil and butter; cook leek, stirring, about 5 minutes or until leek softens. Add garlic, sugar and vinegar; cook, stirring, about 8 minutes or until leek caramelises.
3.Cut eight squares from each pastry sheet; press one pastry square into each pan hole. Spoon leek mixture into pastry cases.
4.Cut cheese into 24 pieces. Place a piece of cheese on each tartlet.
5.Bake tartlets about 20 minutes. Serve tartlets topped with thyme.