A question about a recipe: Plum and Blueberry Custard Tart with a Hazelnut Crust

If your plain yogurt is not "Greek " style, it might have too much water content. If you have just plain yogurt and enough of both time and yogurt, drain the yogurt though cheese cloth or a very fine sieve. If you can, drain overnight. You will lose as much as 50% volume so you will need to start with more yogurt than called for. Then, add any flavor of jam or preserves to sub for the blueberries.

Plain greek yogurt and some macerated blueberries will work. Maybe a 50-50 sour cream and plain non-greek yogurt maybe a better substitute, if that's all you have