1/2 cup water
3 tsp lemon juice
1 To make the paste, wash the quinces, peel then cut into pieces and put into a saucepan. Add the water and lemon juice. Cover and cook over a moderate heat until the fruit is soft, approximately 30 minutes.
2 Put into a food processor and pulse until smooth. Weigh this and add three-quarters of its weight in sugar. Return to the pan and cook over a moderate heat, while stirring often until the quince is a deep pink and the paste leaves the side of the pan when stirring. Pour into an oiled pan lined with greaseproof and leave to cool and dry out.
3 Brush olive oil on to rounds of walnut toast then top with the paste, manchego cheese and a grind of pepper.