Heat oil in a saucepan on medium. Pan-fry onion and garlic for 3 minutes. Brown lamb in batches. Mix in paste and spices and cook for 2 minutes. Stir in tomato and ¼ cup water. Simmer for 1 hour, uncovered, until lamb is tender. Stir in spinach. Cool.
Preheat oven to 180°C or 160°C fan.
Lay a pastry sheet flat. Brush with butter and fold in half lengthways. Place 2 teaspoons of curry in centre of short end. Brush edges with butter. Fold in edges and roll. Place on a baking paper-lined tray. Brush with butter. Repeat with remaining ingredients.
Bake for 20 minutes, until golden. Serve with tzatziki.