- Heat oven to 350 degrees.
- Mix flour, baking powder, baking soda, coconut sugar, and salt in a large bowl.
- Combine egg, milk, and oil, lemon zest and vanilla extract in a small bowl; stir into flour mixture with a fork JUST until moistened.
- Fold in rhubarb and sliced strawberries.
- Fill paper lined muffin tins 2/3 full with batter.
- Sprinkle tops generously with coconut sugar.
- Bake until golden and a toothpick comes out clean, about 22- 25 minutes.
- Cool before serving.