Add cashews and dates to a food processor and process until the mixture starts to come together in a ball and can be pressed together. The cashews should be pretty finely processed,
Line a 12 cup mini muffin tin with paper cases.
Evenly split the date and nut mixture between the tins and using your fingers, press the mixture so that it covers the bottom and goes right up the sides of the paper cases. Put the tin in the fridge for 4 hours or overnight or in the freezer for about 1 hour.
Remove from the fridge/freezer and carefully peel the paper cases away.
Fill each tart with greek yoghurt and top with berries.
* The unfilled tarts will keep in the fridge for 5 days and can be filled and served whenever you need them. Once filled, they won't keep longer than 1 day before they go soft.