Wash and sterilize jars and a knife in hot soapy water or a dishwasher.
Dissolve 200g sugar in 400ml water over gentle heat, then boil for 2 minutes.
Pack plums into jars and pour hot syrup over them, leaving a 2.5cm gap.
Boil the sealed jars in a deep pan of water for 20 minutes then cool.
Store the preserved fruit in a cool, dark place until required.
Rub together flour, icing sugar, and 250g butter until the mixture resembles breadcrumbs.
Bind the dough with an egg yolk and cold water, then chill for 10 minutes.
Preheat the oven to 180°C.
Roll half the pastry to 3mm thickness and line the tart tin.
Cream 100g butter and caster sugar, then beat in the egg and ground almonds.
Spread the frangipane into the pastry case and bake for 30-40 minutes.
Drain plums, cut into wedges, and arrange them over the baked tart.
Reduce the plum liquid by half in a saucepan over high heat.
Brush the reduced syrup over the fruit to glaze the tart.