Preheat the oven to 400 F. In a large bowl, combine the flour, sugar, baking powder, salt and orange rind. In a seperate bowl, beat together the egg, milk and oil. Stir the wet ingredients into the dry ingredients. Gently stir in the rhubarb and strawberries. Do not overmix. Fill 12 well-greased muffin cups 2/3-full with the batter. Sprinkle the muffin tops lightly with sugar. Bake for 20 to 25 minutes, or until a toothpick inserted in the centre of a muffin comes out clean.