Preheat the oven to 375. Heat cast iron skillet over medium heat, add olive oil. When oil shimmers, add mushrooms and cook down 3-5 mins. Add salt to taste. To mushrooms, add breakfast sausage. Cook for 5-7 mins breaking into small bits. Once sausage is completely cooked through and no pink remains, add kale and cook an additional 2 minutes. Allow sausage mixture to cool 15 minutes.
Meanwhile, prep your crust. On a lightly floured surface, roll out the thawed dough. Using a biscuit cutter (or mason jar!) cut circles of dough and place on a parchment lined baking sheet, makes about 10 circles. Scoop 2-3 tbsp of the breakfast sausage mixture to five circles of crust, top with 1 tbsp shredded cheese. To close each hand pie, brush the edges with the whisked egg, place a solo crust circle top and press down with your fingers to seal. Using a fork, crimp the edges to ensure the meat does not leak out while baking. Brush the tops with remaining whisked egg and sprinkle with everything bagel seasoning. Cut small holes into the top of each hand pie to allow the steam to escape.
Bake the pies for 26-28 minutes. Remove from the oven, top with remaining cheese and return to the oven for 3-5 minutes until golden and the cheese has melted. Remove from the oven and allow to cool 5 minutes before serving.