Heat oven to 190C fan, GM6. Line a baking tray with baking paper.
Mix together the flour, sumac, baking powder and black pepper. Add the solid coconut oil and rub into the flour, forming breadcrumbs.
Mix together 4 tablespoons of the dairy-free milk, and the teaspoon of lemon juice with the cold mashed potato. Add to the flour mixture and bring together with a fork. You may need to add up to 3 further tablespoons of dairy-free milk to form a soft dough.
Shape the dough into a circle about 16 cm in diameter and place on the lined baking tray. Mark into six wedges with a sharp knife. Cook for 30-40 minutes, depending on your oven. It should be slightly brown around the outsides.
Allow to cool for 5-10 minutes. Break into wedges along the pre-marked cuts and enjoy warm or cold. Will keep for up to 3 days in an airtight container, and can be reheated for about 5 minutes in a medium oven.