Preheat your oven to 400°F. In a large bowl, combine the flour, brown sugar, baking powder, and salt. Add in the grated butter and mix with your hands until it's about pea-sized and crumbly. Add a cup of the milk and stir until a dough begins to form, adding the remaining ¼ cup as necessary- you may not need all of it!
Dump the dough into a floured surface and work it gently with your hands into one cohesive ball. Roll out into a rough 9" x 13" rectangle, making sure nothing is sticking.
Spread the apples into an even layer on half of the dough, and then add a layer of the streusel on top. Fold the other half of the dough on top of the filling and gently seal the edges so that nothing falls out while you cut the biscuits.
Using a sharp knife or bench scraper, cut the dough into 8 biscuits. Carefully transfer each biscuit to a parchment paper-lined baking sheet. Brush the tops with milk and sprinkle generously with the raw sugar.
Bake for 20-22 minutes, until the tops are golden brown and crisp. Allow to cool slightly before glazing and serving.