Preheat cast iron skillet in a 375 °F oven.
Combine chopped rhubarb and berries to large bowl with sugar, lemon juice, and cornstarch in medium size mixing bowl. Set aside.
Whisk together APF, baking powder, salt, and 1 tbsp of sugar in a medium size mixing bowl.
Add in chunks of butter and using a pastry blender, cut butter into flour mixture until it resembles pea-size pieces.
Pour in sourdough discard and milk.
Mix in the liquids until just combined, you may need to use your hands to gently knead the dough together.
Turn out biscuit dough onto a lightly floured surface.
Gently roll out dough to a 10" circle (or whatever size/shape your pan will be) that is even in thickness.
Remove cast iron pan from oven.
Grease pan with butter or oil - be careful of the hot pan.
Pour rhubarb berry filling into the bottom of the pan - make sure to scrape bowl with spatula to get all the juices of the compote.
Place biscuit crust on top of fruit filling. Crimp edges as desired. Cut 3 slits into the biscuit topping to all steam to escape.
Brush half-n-half on top of dough and then sprinkle with remaining 1 tbsp of sugar as desired.
Place back in oven to bake for 35 minutes or until crust is golden brown and filling is bubbling.
Remove from oven and allow to sit for 10 minutes before serving.
Serve with a scoop of Vanilla Bean ice cream. Enjoy!