Heat oven to 400F. Line a 12-cup muffin tin with paper liners. Mix flour, baking powder, sugar and salt in a large bowl. In a smaller bowl, combine egg, milk and oil; stir into flour mixture just until moistened. Fold in rhubarb and sliced strawberries. In a small bowl, mix together cinnamon and sugar for topping. Spoon batter evenly into the prepared muffin cups, filling about 3/4 full. Sprinkle tops generously with cinnamon-sugar mixture. Bake 20-25 minutes, or until a toothpick inserted in the centre of the muffins comes out clean. Remove muffins from tin and place on a wire rack to cool. Makes 12 muffins.