Preheat the oven to 190°C, 170°C fan, 375°F, gas 5. Start with the filling. Heat the olive oil in a frying pan, add the onion and cook gently for 5 minutes, until beginning to soften. Add the beef and carrots and the thyme, if using. Turn up the heat and cook for a further 5 minutes, until the beef is browned. Set aside to cool completely.
While the filling cools, unroll the pastry on a lightly floured board and cut out 6 rounds with a plain, 11cm (4.5 in) biscuit cutter. Reroll the scraps of pastry and cut out 2 more rounds to make a total of 8. Space out on a baking sheet lined with nonstick baking paper and chill in the fridge for 10 minutes.
Place a heaped tablespoon of filling on one side of each pastry round. Paint a little beaten egg around the edge with a pastry brush. Fold the pastry over to form a semi-circle, pressing the edges together firmly. Use a fork to crimp the edges or pinch them between thumb and finger to form a pattern, if liked. Cut a small slit in the top of each pasty. Repeat until all 8 pasties are made. At this point the pasties may be chilled for up to 24 hours or frozen for up to 3 months. Before baking, paint the pasties with egg to glaze. Bake for 15-20 minutes, until golden brown and puffed up. To make the salsa, combine all the ingredients. Season to taste. Eat the pasties warm or cool, with the salsa.
Tip: Half a teaspoon of 5-spice powder added to the pan with the beef makes a very nice substitute for the thyme leaves, but only add it if your little ones are keen on exotic flavours.
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