To make the tart dough, mix the flour, sugar, and salt in the bowl of a stand mixer fitted with the paddle attachment. (You can also make it in a food processor, or by hand, using a pastry blender.) Add the cold butter and mix until the butter pieces are the size of peas.

Add the ice water and continue to mix just until the dough comes together. Gather the dough with your hands, shape it into a disk, wrap it plastic, and refrigerate for at least 30 minutes.

Put the rhubarb and strawberries in a medium bowl with the lemon zest. Sprinkle the sugar and corn starch on top, but do not mix the ingredients together yet. (If you do, they'll start to juice and may be overly juicy by the time you're ready to use them.)

Preheat the oven to 400ºF (200ºF). Line a baking sheet with parchment paper or a silicone baking mat.

On a lightly floured surface, roll the dough to a 14-inch (36cm) circle and place it on the baking sheet. Sprinkle the almond flour over the tart dough, if using. Mix the fruit together with the sugar and corn starch and place the fruit into the center of the tart dough, then spread it with your hands, leaving 3-inch (8cm) of space between the fruit and the edge of the dough.

Fold the edges of the dough up and over the fruit filling. Brush the crush liberally with melted butter and sprinkle with sugar. Bake the tart until the filling is cooked and bubbling, and the crust is golden brown, 35 to 45 minutes. Remove from the oven and slide the tart off the baking sheet, onto a cooling rack.