Cut butter and shortening into small cubes and freeze 30 minutes.
Add flour to a food processor. Add butter and pulse 3 or 4 times. Add shortening and pulse 3 or 4 times. (Short pulses.)
Add salt and pulse three times at three seconds each time. (yes, I count.)
Pour water, one tablespoon at a time into processor, pulsing after each addition.
After all the water is in, pulse until dough gets crumbly, but stop before it forms a ball.
Lightly flour plastic wrap (like Saran wrap) and turn the mixture onto the plastic wrap. Bring the dough together with your hands. Form the dough into a disc, seal tightly and refrigerate for at least 30 minutes.
PIE FILLING
Meanwhile, Place blueberries in a bowl. Cube mango and place with blueberries.
Stir in lemon juice and zest.
Add sugar and flour.
Stir and allow to sit at room temperature until pie crust is ready.
TO MAKE CRUST
Roll the dough out on a cool surface as quickly as possible to keep the dough from melting.
Roll evenly. Measure the diameter by the pan you plan to bake in. I used a 8.5 inch cast iron skillet.
ASSEMBLE
Transfer the crust to the skillet with the help of your rolling-pin if necessary. Leave the crust hanging over the edge until you fill it.
Spoon the blueberry mango mixture onto the crust. Gently fold over the edges. Brush with crust with the egg white.
Sprinkle a pinch of salt over cobbler.
Bake at 350 degrees for 45 to 55 minutes or until berries are cooked and crust is brown.