Preheat the oven to 200°C (400°F), 180°C (350°F) for fan assisted.
Add the cauliflower florets to a pan of boiling water and simmer until tender. The cauliflower needs to be soft enough to blend but not overcooked.
Whilst the cauliflower is cooking, heat one tablespoon of olive oil in a large pan over a low/medium heat. Add the onion, garlic, celery and the sprig of rosemary. Sweat the vegetables in the oil until tender.
Add another tablespoon of oil to the pan and add the lamb mince. Season with a generous pinch of salt and pepper. Cook over a medium heat until browned all over.
Add the chopped tomatoes, balsamic vinegar and hot stock to the pan. Bring to a gentle boil then simmer until the liquid has reduced and thickened – about 15 minutes.
Whilst the lamb cooks, transfer the cauliflower to a food processor with the chopped rosemary, parmesan, butter and remaining olive oil. Season with a good pinch of salt and pepper and blend until smooth and creamy.
Remove the rosemary stalk from the filling and transfer the cooked lamb mixture to an oven proof dish. Spoon the cheesy mash evenly over the surface.
Transfer to the oven to bake for 25 minutes until cooked through and the mash is lightly golden all over.