Preheat the oven to 220℃ (200℃ fan)/425°F/gas mark 7. Wash 1½kg plums and cut in half discarding any loose stones and place in a single layer on a large baking tray.
Measure 1kg sugar and sprinkle 2-3 spoonfuls of the sugar over the plums. Roast in the oven for 15 minutes.
Lift off any plum stones with a fork and discard. Tip the roasted plums and the juices into a preserving pan or very large saucepan and add the remaining sugar.
Cook gently over a low heat stirring until the sugar has dissolved.
Bring to a rolling boil and boil for 6 - 8 minutes, then test for a set. If a set has not been reached continue to boil for another 2 minutes before retesting. Repeat if necessary until setting point is reached. If you have a thermometer the mixture should reach 105℃/220°F.
Pour into hot sterilised jars and cover. Label when cool.
Place a couple of sauces into the fridge before starting, ready for testing the jam for setting point or use a thermometer.Check out my How to post for more preserving tips.Nutrition information is calculated based on this jam making 4 x 450g (1lb) jars and each serving being 25g (1oz)