Preheat the oven to 200C/180C Fan/Gas 6. Tip the blueberries and cassis into a wide pan set over a medium heat. Heat until the blueberries start to pop, then transfer everything to a bowl.
Sprinkle the almonds over the base of the tart shell. Add the warm blueberries, reserving some to decorate.
Combine all the custard ingredients together in a large jug. Mix well, then pour the custard over the blueberries in the tart shell. Bake in the oven for 25 minutes.
Serve the tart in slices with the reserved warm blueberries.
