Sprinkle the raspberries with caster sugar and leave in a colander to sweeten and drain at the same time. Place the drained raspberries in a single, tightly packed layer in the base of the tart shell. Very carefully put splodges of the frangipane over this – do not press it down. Do not cover completely, the splodges will melt and join up over the fruit.
Bake on a baking sheet until lightly brown and risen, about 30 minutes. Cover with flaked almonds and icing sugar. Turn the