If you are preparing your own pastry crust, follow the steps indicated in the recipe found at the end of the link and then let rest. If you use a store-bought pastry crust, skip to the next step below.

Peel all the apples. Slice them in thin slices (about a quarter or a Euro coin thickness). If you choose to use rhubarb instead of apples, follow the same steps for the rhubarb.

Put all the sliced apples into a non-reactive bowl. Mix about half of the tapioca flour or cornstarch with about half of the sugar and add to the apples. Add seasoning. Let rest and mix while you prepare the pie crust.

Preheat oven to 180°C / 350°F. Roll out your homemade or store-bought pastry crust into a pie tin. Fold over the top of the crust into your preferred pattern. I usually just crumple the dough using my forefinger and thumb going around the top edge. Make sure you prick the bottom of the pie crust several times.

Put the remainder of your tapioca flour, flour or cornstarch in a bowl. Remove the frozen berries from the freezer and add them to the bowl. If you use fresh berries, add them now following the same steps. Stir them gently into the tapioca flour, flour or cornstarch so all the fruit is lightly coated. This will help the fruit keep their form as much as possible.

Add the remainder of the sugar to the berries. Mix the berries into the bowl with the apples or rhubarb. Add the lemon juice and let sit a couple minutes.

Place the fruit into your pie tin. You can either do this in attractive manner by making a design or simply by pouring the fruit in. Either way, I usually fix the fruit a little so the fruit is evenly placed around the pie and it looks as attractive as possible.

Put little pieces of butter (fingertip size) randomly on top of the pie. If you prefer you can push them down a bit into the fruit.

Place the pie dish in the top half of the oven for approximately 55 minutes or until the crust is golden and the fruit looks cooked. I usually turn my pie half way and then gauge my time from how it looks at that moment. Start checking it though after 40 minutes or so to prevent from burning. Everyone's oven is different. Sometimes if the crust looks like it is browning too early, I will place at this time a sheet of foil loosely on top of the pie (not folded down onto the pie) to help prevent the crust from burning before the fruit filling is done.

Remove the pie from the oven and let cool before serving. It can be served warm (not hot) with ice cream or at room temperature however you wish. The pie might be more liquidy if you serve it warm than if it is cooled completely.