Preheat oven to 200°C/180°C fan-forced. Line 1 baking tray with baking paper.
In a mortar and pestle grind the onion and garlic into a fine paste. Add cumin, coriander, sumac, chilli flakes, salt, Harissa and sweet paprika.
In a large mixing bowl add mince and onion mixture and work vigorously until well combined. Shape 1/2 cup mince mixture into a sausage shape. Place along edge that is closest to you. Brush opposite edge with a little of the eggwash. Roll up pastry, starting from edge that is closest to you, to enclose filling. Cut into 4 pieces. Place, seam-side down, on prepared trays. Repeat with remaining mince mixture, pastry and egg to make 24 rolls.
Brush pastry tops with remaining eggwash. Sprinkle with nigella seeds. Bake for 25 to 30 minutes, or until golden and puffed. Stand for 5 minutes.
Meanwhile combine 1 tbsp Harissa paste with aioli in a small bowl. Serve sausage rolls with sauce.