- Place the diced lamb, flour, nutmeg and salt and pepper into a bag or bowl and toss to make sure all the lamb is well-coated.
- Heat the oil in a medium-heavy pot over medium-high heat. Cook lamb until browned on all sides, about 8 minutes. Add the beer and stir well to combine.
- Bring to a simmer. Cover, and reduce heat to medium-low. Gently simmer until lamb is tender, about 1 hour, stirring at intervals to avoid sticking.
- Stir in the prepared mushrooms, adjust seasonings to taste. Allow to simmer for 10-15 minutes with the lid removed, until the juices are reduced and thickened. Allow to cool completely.
- Divide stew among 4 small pie dishes.
- Cut four 7-inch squares of pastry. Cover each pot pie with a pastry square, allowing it to drape over the sides. Refrigerate until cold, approximately 15 minutes.
- Preheat oven to 400°F. Brush pie tops with egg wash and sprinkle with black sesame seeds. Bake pies on a rimmed baking sheet until crusts are golden brown, about 30 minutes. Transfer to a wire rack to cool slightly, about 10 minutes.
- Serve with creamy mashed potatoes and vegetables of your choice.