To make the pastry, whizz the flour and margarine in a food processor (add a tiny splash of cold water if you need to bring it together). Wrap in clingfilm and leave to rest in fridge while you cook the filling.
Heat a large pan with olive oil, add the onions and cook until golden and caramelised. Dust the lamb pieces in flour, then add to the onions and cook until browned.
Add the garlic, all the herbs, red wine, about half the stock and a pinch of cracked black pepper. Bring to the boil and then turn down to a simmer and leave for 11⁄2-2 hours, stirring occasionally to prevent sticking and adding a little more stock if needed.
Once the lamb is tender (if it collapses fairly easily it’s ready), add the vegetables. Cover with the remaining stock and simmer for 10-15 minutes or until the veg is tender. Thicken with the gravy granules and finally add the mint (or mint sauce), taste and season.
Heat the oven to 200C/180C fan/gas 6. Roll out the pastry and use 2/3 to line a 20cm x 5cm deep pie dish. The pastry is quite soft so just patch it up if it breaks.
Add the filling being careful not to overfill. Roll out the rest of the pastry for the lid. Trim and seal by crimping around the edge with your fingers. Egg wash the pie and allow to dry. Egg wash again, sprinkle with dried mint (this is how we identify the pies at the pub) and bake for 30-40 minutes, or until crisp and golden.
Recipe from The Chestnut Horse pub at time of publication (summer 2014), now called The Piebald Inn