"The rhubarb and blueberry are so good together here, but do play around with the fruits, if you like; figs and raspberries also pair nicely. If you’d prefer to use just one fruit, some thinly sliced peaches, apricots, or apples work well. Serve this with whipped cream or vanilla ice cream alongside.
The Amaretti cookies are for soaking up the excess liquid from the fruits. You want the dry, crumbly variety, not the soft chewy ones. They’re not hard to get hold of, but if you find yourself without, crushed ladyfingers or ground almonds can be used instead. They won’t provide the flavor hit of the Amaretti, but will be fine."
To make the cream cheese pastry: Cover the cubes of butter in plastic wrap and place in the freezer for about an hour to freeze solid. Place the flour, salt, and baking powder in a resealable freezer bag and freeze for 30 minutes.
Tip the flour mixture into the bowl of a food processor and process for about 30 seconds to combine. Add the cream cheese and process for another 20 seconds, or until the mixture has the consistency of coarse breadcrumbs. Add the frozen butter cubes and pulse to form crumbs. They’ll be uneven in size—some the size of peas and some a bit larger—but that’s fine.
Add 2 tablespoon of the heavy cream and the vinegar and pulse until the dough starts to hold together; add the remaining 1 tablespoon cream if the dough needs it. Tip onto a clean work surface and use your hands and knuckles to press the mixture until it holds together in one piece. Cover the pastry loosely in plastic wrap (or put it in the bag you used to chill the flour) and press to flatten it into a disk. Place in the fridge for 45 minutes (or up to 2 days).
Preheat the oven to 400°F/200°C.
To make the base and filling: Roughly crush the Amaretti cookies in a small bowl. Add the granulated sugar, flour and cinnamon and set aside.
Place rhubarb, blueberries, ½ cup granulated sugar, 1 tablespoon tapioca flour or cornstarch, orange zest, and ⅛ teaspoons salt in a medium bowl. Toss gently to combine and then set aside.
Remove the disk of pastry rom the fridge 10 minutes before you want to roll it, so that has been lightly dusted with flour. Roll out evenly into a large circle, about 15 inches wide and ⅛ inches thick. Trim edges to create a rough circle, then transfer it, along with the parchment paper (it is too soft to transfer alone), onto a large baking sheet. Sprinkle the base in a 10-inch circle over the middle of the pastry. Spoon the fruit filling on top of the crumbs and then carefully draw the pastry border up and over the fruit-filled center of the galette exposed. If the pastry has become too soft or warm during the process, place the galette in the fridge to chill for up to 1 hour before baking.
**To make the glaze**, beat the egg with the water in a bowl and brush all over the outside of the pastry, then sprinkle with the sugar.
Bake for about 40 minutes, rotating the baking sheet halfway through, or until the galette is golden. Remove from the oven and set aside on a wire rack to cool before serving.