In a food processor, add all the crust ingredients and blend until the vegan butter is broken down into tiny bits and a dough forms. If the dough is too crumbly, you can add a bit of water.
Line a baking sheet with parchment paper and turn the dough onto the paper. Sprinkle a bit of flour to prevent the dough from sticking and work it into a ball with your hands. Roll it out into a somewhat round shape that is ⅛" thick; don't worry about the jagged edges.
In a medium saucepan, cook the rhubarb, strawberries, and sweetener of choice for about 5 minutes, until it starts to soften. Stir in the corn starch slurry and cook for another 1 to 2 minutes to thicken up. Turn off the heat and stir in the vanilla extract.
Transfer the cooked filling mixture to the center of your rolled out crust, leaving a 2" border. Then fold the border over on top of the filling. If the edges crack open, just press it back together with your finger tips. Brush the crust with olive oil and sprinkle with turbinado sugar, if desired.
Bake in the preheated oven for 35 to 40 minutes, or until the crust is golden brown and the filling is bubbling. Cool for at least 10 minutes before serving.
Notes
*if using granulated sugar in the filling, add 2 tbsp of water to the pot to assist cooking
Recipe by Radiant Rachels at https://radiantrachels.com/rustic-vegan-strawberry-rhubarb-galette/