For the topping
- STEP 1
Put the oil in a frying pan and cook the onion until soft. Add the carrot, celery and garlic and cook until lightly browned. Mix in the mince, breaking it up with a spoon. Cook, stirring often, until the meat is browned. Add the thyme, tomatoes, stock, Worcestershire sauce and parsley, and season to taste. Cover tightly and simmer for 15 minutes. Preheat the oven to 200C/gas 6/fan 180C.
- STEP 2
Sift the flour and baking powder and rub in the butter. Add the cheese and herbs, then the yogurt, and mix to a soft dough. Turn out on to a lightly floured board and knead lightly. Roll to about 1cm/1⁄2in thick and cut out 9 rounds.
- STEP 3
Tip the mince mixture into an ovenproof dish. Casually arrange the dough circles on top of the mince and brush with egg or milk. Bake for about 20-25 minutes until the topping is risen and golden brown and the filling is bubbling.