Spray a 9 x 13-inch baking pan with cooking spray. Tear French bread into chunks, or cut into cubes, and evenly place in the pan.
In a large bowl, mix together eggs and milk. In a small bowl, combine sugar and orange zest. Rub together with your fingers until fragrant. Add sugar mixture to the eggs and milk. Add in vanilla extract and cinnamon. Stir until combined. Pour evenly over bread. Combine strawberries and rhubarb together in a medium bowl. Pour evenly over French toast. Take a spoon and poke the strawberries and rhubarb down into the bread. Cover the pan with saran wrap and store in the refrigerator for several hours or overnight.
In a separate bowl, mix together flour, brown sugar, cinnamon, and salt. Add butter pieces and cut into the dry mixture using a fork or your hands. Combine until the mixture resembles sand with a few pea sized chunks. Cover and store mixture in the refrigerator.
When you’re ready to bake the French toast, preheat oven to 350 degrees F. Remove French toast from refrigerator and sprinkle crumb mixture over the top. Bake for 45-55 minutes, or until the French toast is set and golden brown.