Lightly cook the lamb in a spot of olive oil in a pan. It should still be about 50% pink. Set aside the lamb keeping the leftover juices in the pan.
Finely chop up the garlic and let cook slightly on medium heat. Roughly chop up the mushrooms in large chunks, approximately 1 inch. Add the mushrooms to the garlic and cook until the mushrooms stop releasing water/steam.
Whisk up 2 eggs and sour cream until combined. Add the fresh chopped parsley, parmesan, and pepper. Mix thoroughly.
Roll out the pastry dough and place it on a cookie sheet with parchment paper. Spread the mustard on half of the pastry, leaving a 1/2 in a border.
Add the mushrooms to the egg mixture and combine until the mushrooms are well coated.
Place the lamb on top of the spread mustard. Next, add the egg and mushroom mixture. Be careful as the egg may run.
Carefully fold over the other side to form a half-circle. This may be tricky, especially if your mushrooms were not thoroughly cooked and watered down the egg mixture.
Fold over the bottom dough over the top and pinch together. You may need to use a little water to act as glue between the edges of the dough.
Once thoroughly pinched together and the calzone is sealed, beat up the remaining egg to use as an egg wash. This will make the pastry dough have a golden and flaky like appearance. Brush over the top of the calzone. Use a fork to poke holes at the top to allow steam to escape.
Place in the middle of the oven for 20 minutes or until the crust is nice and golden.