1/2 cup water
3 tsp lemon juice
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.
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.
Brush olive oil on to rounds of walnut toast then top with the paste, manchego cheese and a grind of pepper.