Preheat the oven to 200C/400F/Gas 6.
Fry the mince in a large frying pan for 4-5 minutes, or until browned. Strain off any liquid.
Heat the oil in a separate frying pan and fry the onions for 4-5 minutes, or until lightly browned then add them to the lamb mince.
Add the vegetable stock, herbs, tomato purée and season with salt and freshly ground black pepper. Cook gently for one hour, or until the meat is cooked through and tender.
Meanwhile, cook the potatoes in boiling salted water for 5-10 minutes, or until tender. Drain the potatoes and mash with the butter and milk.
Place the meat in an ovenproof casserole dish and spread the potatoes on top.
Brush the beaten egg over the top of the potatoes. Run a fork over, then arrange the sliced tomatoes on top. Bakee the pie in the oven for 25-30 minutes, or until bubbling and golden-brown on top.