1. Preheat your oven to 350 degrees Fahrenheit. Lightly grease 12 medium-sized muffin cups and set aside.
2. In a medium bowl, combine the soy milk and vinegar, and let sit for a minute to curdle. Add in the sugar, oil, and vanilla, stirring thoroughly until the mixture is homogenous.
3. Separately, mix together both flours, baking powder and soda, cardamom, and salt, making sure that all the ingredients are completely incorporated. Mix in the rhubarb and strawberries, tossing to coat. By covering them with flour, you will prevent the fruits from sinking to the bottom of your muffins, so make sure you don’t skip this step!
4. Pour the wet ingredients into the dry, and stir just until combined. Distribute the batter evenly between your prepared muffin cups and top them with a sprinkle of both the oats and turbinado sugar.
5. Bake for 18-24 minutes, until a toothpick inserted into the center comes out clean. Let cool in the pan for 15 minutes before turning them out and cooling them completely on a wire rack.