Method:-

• Melt the butter and gently fry onions and sugar for 30 minutes until well softened
• Add the vinegar and thyme and simmer for another 5 minutes until the liquid has almost gone
• Leave to cool
• Pre-heat the oven to Gas Mark 7
• Unroll the puff pastry, roll thinner if need be
• Cut into 4inch rectangles
• Place on baking sheet and prick with a fork
• Bake for 10 minutes then remove from oven and flatten with a spatula
• Put onion mixture on pastry
• Put cheese slice on top
• Season with black pepper
• Bake for 6 to 8 minutes until pastry is golden brown and the cheese is melting