Preheat the oven to 350°F and spray a bread loaf pan with cooking spray. (You can also line with parchment paper for easier removal.)
In a large bowl combine the flour, baking powder, baking soda, sugar, salt and cinnamon.
In a separate bowl beat the egg and add the oil, lemon juice, vanilla, and buttermilk. Pour into the dry ingredients and mix just until combined (don't over mix).
Add the rhubarb and strawberries and fold in gently, then pour into the bread pan.