Preheat your oven to 375°F (190°C) and line a standard 12-cup muffin tin with paper liners or grease well with cooking spray.
Prepare your fruit by washing the strawberries and rhubarb thoroughly. Remove the strawberry stems and dice into roughly ¼-inch pieces. For the rhubarb, trim away any leaves (which are toxic) and dice the stalks into similar-sized pieces.
Whisk together all dry ingredients in a large bowl: flour, sugar, baking powder, baking soda, salt, and cinnamon until well combined.
In a separate medium bowl, combine all wet ingredients – vegetable oil, egg, Greek yogurt, and vanilla extract. Whisk until smooth and no yogurt lumps remain.
Pour the wet ingredients into the dry ingredients and gently fold together using a spatula just until combined. Be careful not to overmix – a few small flour streaks are perfectly fine.
Gently fold in the diced strawberries and rhubarb using just a few strokes to distribute evenly without crushing the fruit or overworking the batter.
Divide the batter evenly among the muffin cups, filling each about ¾ full. Sprinkle the tops with coarse sugar if using.
Bake for 22-25 minutes until the tops are golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Cool in the pan for 5 minutes before transferring to a wire rack to cool completely.