Make the dough. In a large bowl, stir 1.5 tsp salt into 1 cup warm water until dissolved. Add the flour and mix with your hand until a shaggy mass forms, then turn out onto a lightly floured counter and knead firmly for 10 minutes, until smooth, tight, and only slightly tacky.
Shape the dough into a ball, cover with plastic wrap or an inverted bowl, and rest at room temperature for 30 minutes. Do not skip the rest — it relaxes the gluten so the dough can be rolled paper-thin.
Soften the lamination fat. If using lamb tail fat, chop it very finely and warm it in a small skillet over low heat just until it turns spreadable and slightly translucent, about 3 minutes. Do not melt it to liquid. If using ghee, simply soften it to the texture of room-temperature butter.
Make the filling. On a large board, hand-chop the lamb shoulder with a heavy knife into pea-sized pieces, keeping the fat mixed evenly through the lean. Scrape into a bowl with the very finely diced onions, 2 tsp salt, the cumin, and the black pepper. Add 3 tbsp cold water and mix vigorously with your hand for a full minute, until the mixture looks glossy and slightly sticky. Refrigerate while you laminate the dough.
Laminate the dough. On a floured counter, roll the rested dough into a very thin rectangle about 20 by 24 inches — thin enough that you can almost see the counter through it. Spread the softened lamb fat or ghee in an even, thin layer all the way to the edges using your fingers or a pastry brush.
Starting from a long edge, roll the dough up tightly into a long, snug log, like rolling a poster. Gently stretch the log as you go to keep the layers tight. Cut the log in half, wrap both pieces in plastic, and chill in the refrigerator for 30 minutes so the fat firms up.
Place a pizza stone or baking steel on the middle rack and preheat the oven to 500 F for at least 30 minutes. The stone must be ripping hot to mimic the tandyr.
Cut each chilled log crosswise into 6 equal pieces, for 12 pieces total. Stand each piece on its cut end so you can see the spiral of laminated layers, and press down gently with your palm to flatten into a thick disk.
Roll each disk into a 5-inch round, working from the center outward and keeping the spiral side facing up and down — never flip the dough or you will crush the layers. The edges should be slightly thinner than the center.
Fill and shape. Place a heaping 2 tablespoons of filling in the center of each round. Lift three sides of the dough up and pinch them together firmly at the top to form a sealed triangle, then pinch each seam closed all the way down. Press lightly on the counter to flatten the bottom so the samsa will sit flush on the hot stone.
Brush the tops generously with beaten egg and sprinkle each samsa with a good pinch of nigella seeds. Arrange them sealed-side down and bottom flat on a piece of parchment.
Bake. Slide the parchment with the samsa directly onto the screaming-hot stone. Bake at 500 F for 12 minutes, then reduce the heat to 425 F and bake for another 10 to 13 minutes, until the tops are deeply golden, blistered, and the bottoms are crisp and browned.
Rest the samsa for 3 minutes on a rack before serving — the filling is volcanic inside. Eat hot, holding one in a napkin and biting the corner to sip the rendered lamb juices first, the way they do in Tashkent.