Heat a large heavy based saucepan over medium heat. Add olive oil, onion, carrot and celery and sauté until softened. Add garlic and rosemary and sauté for further 30 seconds.
Add lamb mince, break up with a wooden spoon and cook until browned.
Add flour and tomato paste, stir through and add wine, stock, bay and Worcestershire sauce and season with salt and pepper. Bring to the boil, then simmer for 30 minutes.
Stir through peas and transfer to a 1.5L baking dish. Set aside to cool slightly. Preheat oven to 200C.
Meanwhile, place potatoes into a medium saucepan, cover with cold water and season with salt. Bring to the boil, then simmer until tender. Drain, return the potatoes to the warm pot, place over low heat and allow the potatoes to steam dry.
Mash potatoes thoroughly with a masher or pass through a mouli. Stir through butter and milk and season to taste with salt.
Spread mash over the filling with a spatula, creating peaks and troughs.
Bake for 30 minutes or until golden and bubbly.
Allow to rest for 10 minutes before serving, garnished with chopped parsley.