Rhubarb Blueberry Custard Tart Questions

Can I use frozen rhubarb or frozen blueberries?

Frozen fruit works, but it releases far more liquid than the fresh rhubarb and blueberries called for here, which can make the custard layer runny. Thaw the rhubarb, drain it well and pat it dry before cutting it into small pieces, and scatter blueberries on while still frozen so they hold their shape. Expect the bake to need the upper end of the 40-45 minutes, or slightly longer - keep an eye on the colour rather than the clock.

Why did my custard turn out lumpy or thin?

Lumps almost always come from adding the warm milk to the egg yolk, sugar, vanilla and cornstarch mixture too fast - pour it in slowly in a thin stream while whisking constantly. A thin custard means the 35 g of cornstarch never fully cooked out: once it is back in the pot it has to actually bubble and thicken, not just steam. If it does go lumpy, push it through a sieve before stirring in the 25 g butter and covering it with cling film.

How far ahead can I prepare the components?

The dough and the custard are both made to be prepared ahead: the dough needs 30 minutes of chilling minimum and can rest in the fridge overnight, and the custard has to cool completely anyway before it goes onto the base. Keep the custard covered directly with cling film so no skin forms, and let it come to room temperature briefly and whisk it smooth before spreading. The crumble of flour, sugar and cold butter can also be mixed a day ahead and kept cold. Only cut the rhubarb shortly before assembling so it does not weep.

How do I store the finished tart and how long does it keep?

Because of the egg yolk custard, the tart belongs in the fridge - it already goes in for a few hours before unmoulding, and it should stay there afterwards. Cover it loosely rather than airtight so the almond crumble topping does not go soft. It is at its best on the first or second day; after that the shortcrust base gradually softens from the moisture in the rhubarb.

What is the most common mistake with this tart?

Cutting into it too early. The instruction to cool it completely without touching it and then refrigerate for a few hours is what lets the custard set firm enough to slice - a warm tart will slump the moment you lift a piece out. The second common mistake is skipping the second chill after lining the pan; a well-chilled, fork-pricked base is what keeps the sides from slumping under the wet custard and fruit.