Step 1In a large, heavy-bottom saucepan over medium heat, melt the butter. Add the chopped onion and cook until softened, about 5 minutes. Stir in the minced garlic and reduce the heat to medium-low until aromatic. Remove from heat. .
Step 2Trim the lamb meat from the bone into one-half-inch pieces (you should end up with about three-fourths pound). Add the lamb to the onion and garlic mix and increase the heat to medium. Cook, stirring occasionally, until the lamb is no longer pink, about 5 minutes.
Step 3Stir in the cubed sweet potatoes, tomato paste, water and harissa to the pan. Reduce the heat to medium-low, cover and steam, just until the potatoes begin to soften, about 5 minutes, stirring occasionally. Stir in the curry powder, cayenne, fennel seed, coriander and tamarind paste, scraping the side of the pan to incorporate any crusty bits. Cook until most of the liquid has evaporated and the potatoes are nearly soft, about 8 minutes. Season with one-half teaspoon salt, or to taste, and pepper to taste, and adjust the heat with additional cayenne if desired. Remove the pan from heat, and set aside to cool.
Step 4Heat oven to 375 degrees. Remove the chilled dough from refrigerator and divide into six equal-sized balls. On a clean, floured surface, press each ball into a flattened disk approximately 6 to 7 inches in diameter.
Step 5Going one at a time, brush the edges of each disk with beaten egg and add a one-third-cup mound of cooled lamb filling to the center. Then fold the dough over to make a half-circle, crimp edges to close and transfer to a baking sheet lined with parchment paper.
Step 6Brush the top of each hand pie lightly with beaten egg. Cut two steam vents in the top of each. Bake the pies until they are a deep, golden brown, about 40 minutes, rotating halfway through for even coloring.