In a large pot over medium heat, melt the butter. Add the minced garlic and diced onion and let cook until the onion has softened.
Whisk in the flour until there are no lumps, and let cook over medium heat until bubbly and lightly golden brown.
Pour in the chicken stock and whisk until smooth. Add the apple cider vinegar, salt, pepper, and rosemary. Turn heat to low and let simmer.
Place the sweet potato and Yukon gold potato into a shallow microwave safe container. Add water until the potatoes are just submerged, and microwave on high for 6 minutes.
Stir, heat 6 minutes more. The potatoes should be fork tender but not mushy. Drain well and set aside.
Remove the sausage from its casing, and in a large skillet over medium heat, cook the sausage until no longer pink. Break apart into small chunks with a spatula as it cooks, and drain off any liquid once the sausage has cooked fully. Turn up to high heat and cook until the sausage starts to brown, stirring occasionally. Set aside.
Stir the cooked potatoes and sausage into the thickened stock mixture. Slice the apple in half and remove the core and seeds. Leaving the skin on, dice the apple into small pieces and add to the rest of the filling. Stir to combine everything, taste to adjust seasoning if desired.
Transfer the filling into a 10-inch cast iron skillet. Fold the pie dough sheet into half and half again, then make two 1-inch diagonal cuts on each side to vent. Cover the pie filling with the unfolded pie dough. Crimp the sides and press into the rim of the skillet to secure the crust.
Use a pastry brush to brush the pie dough with milk. Optional: cut an \"X\" shape into the middle of the pie and insert a pie bird to prevent spillage.
Bake the pot pie in the preheated oven for 40-55 minutes, or until the filling is bubbling and the crust is golden brown. Allow the pie to rest for 5-10 minutes before eating.