Place flour, salt, melted butter and the water in a large bowl; stir with a round-bladed knife until a firm dough forms. Knead dough on a lightly floured surface for 2 minutes or until smooth. Divide dough into three portions; shape into discs, wrap each in plastic wrap. Refrigerate 1 hour.

To make the filling, heat half the oil in a large frying pan over low heat; cook onion, stirring occasionally, for 10 minutes or until soft. Add spices; cook, stirring, for 2 minutes or until fragrant. Transfer to a large heatproof bowl. Meanwhile, trim any fat from beef; cut beef into 1cm (½-inch) pieces. Heat remaining oil in same pan over high heat; cook beef, stirring, in batches, for 2 minutes or until browned, reheating the pan between each batch. Add beef to onion mixture with oregano, egg and olives; season to taste, then stir to combine. Stir in butter until well combined. Refrigerate 1 hour or until cool.

Preheat oven to 200°C/400°F. Line two large oven trays with baking paper.

Roll out one piece of dough on a lightly floured surface until 2mm (inch) thick; cut out 10 x 8cm (3¼-inch) rounds. Knead and re-roll scraps, taking care as the dough may become tough and shrink on re-rolling. Holding a round of dough in one hand, place 1½ teaspoons of filling in the centre. Fold dough over the filling; pleat edges to seal. Place on tray. Repeat with remaining dough and filling. Brush tops lightly with egg.

Bake empanadas for 20 minutes or until browned lightly. Serve warm with tomato salsa.