Make the crust: In a food processor, pulse the chilled coconut oil, flour, salt, and 1 tsp (5 ml) Sucanat until the mixture resembles coarse sand with some pea-sized pieces remaining.
Add the ice water 1 Tbsp (15 ml) at a time, pulsing, just until the dough begins to stick together when pressed between your fingers. Be careful not to overmix.
Transfer the dough to a large piece of plastic wrap.
Form it into a ball, then flatten it into a disk.
Wrap tightly and refrigerate for at least 30 minutes.
Prepare the filling: In a large bowl, stir together the rhubarb, cherries, mixed berries, 3 Tbsp (45 ml) Sucanat, arrowroot, ¼ tsp cinnamon, and nutmeg until well combined.
Set aside.
Preheat & Prep: Position a rack in the center of your oven and preheat to 350°F / 177°C.
Line a large baking sheet with parchment paper.
Roll out the dough: Remove the chilled dough from the refrigerator.
Place it on a lightly floured piece of parchment paper. Dust the top of the dough and your rolling pin with flour, and roll it out into a 12-inch circle, about 1/8-inch thick.
Transfer the parchment with the dough onto your baking sheet.
Assemble the crostata: Mound the fruit mixture in the center of the dough, leaving a 2-inch border all around. Gently fold the border of the dough up and over the filling, creating pleats as you go. The center of the fruit will remain exposed. Pinch any cracks in the dough to seal them.
Finishing touches: Brush the pleated crust with plant-based milk.
In a small bowl, mix the remaining 1 tsp (5 ml) Sucanat and the pinch of cinnamon, then sprinkle it over the crust.
Bake for 50 to 60 minutes, or until the crust is golden brown and the fruit filling is tender and bubbling.
Cool & Serve: Let the crostata cool on the baking sheet for about 10 minutes.
Then, carefully slide a metal spatula underneath to loosen it and transfer it to a cutting board or serving platter.
Slice with a sharp knife and serve warm.
Don't overwork the dough. For a tender, flaky crust, pulse the until just combined. The pea-sized pieces of cold coconut oil are key to creating flaky layers as they melt during baking.