In a large, deep skillet, heat the olive oil over moderate heat. Add the peppers, pul biber and onions, season with salt and pepper and cook, stirring frequently, until softened, about 10 minutes. Add the beef and cook until the liquid has evaporated, about 10 minutes. Season with salt and pepper. Continue cooking over moderate heat, stirring occasionally, until the vegetables are softened, about 15 minutes. Transfer the vegetables and meat to a bowl and let cool slightly.
Preheat the oven to 400 degrees F and lightly oil a 9- by 13-inch glass baking dish.
In a medium bowl, whisk the eggs with the milk and 1/2 cup of the canola oil. Lay a sheet of yufka on a work surface and brush with the egg mixture; repeat until you have a stack of 2 sheets. Transfer the stack to the baking dish and spread one-third of the beef mixture on top. Repeat the layering twice more with the brushed yufka sheets and beef filling, then end with a stack of brushed yufka.
Trim the overhanging yufka slightly and tuck it under itself. Brush any remaining egg mixture on top of the borek, top with the nigella sativa seeds and bake until deeply golden on top and sizzling, about 45 minutes. Let the borek rest for 20 minutes before cutting into squares and serving.
Recipe courtesy of Scott Conant