1 1/2 teaspoons black onion seeds, plus extra for sprinkling
1 large egg, lightly beaten
2–3 tablespoons natural yogurt
Line 1 large baking sheet with nonstick baking paper. Sift the flour, salt, baking powder and mustard into a large bowl. Add the butter and rub in with the fingertips until the mixture resembles fine breadcrumbs. Stir in the onion seeds.
Whisk together the egg and yogurt in a jug, reserving 1 tablespoon to glaze. Pour into the dry ingredients and mix to form a soft dough.
Turn the dough out on to a floured surface and roll out to 1.5 cm (3/4 inch) thick, then stamp out 7 or 8 rounds using a 6 cm (2 1/2 inch) plain cutter, without twisting the cutter. Place on the prepared baking sheet and brush with the reserved glaze, then sprinkle with onion seeds.
Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10–12 minutes until risen and golden. Serve warm.