1. To make potato purée, place potatoes in a stockpot of lightly salted, cold water. Bring to boil over high heat. Reduce to low and simmer for about 12-15 minutes, or until tender. Drain well, then return to pan and shake over a low heat to allow any excess moisture to evaporate. Remove. Push potato through a ricer or fine-mesh sieve into a bowl. Stir in butter and warm milk. Season with salt and pepper.
2. Meanwhile, place turkey in a bowl with oil and garlic, then season with salt and pepper. Toss to coat.

3. Place prosciutto on a clean board and top each slice with turkey. Fold one short side over, then top with sage leaves and secure with toothpicks.
4. To cook turkey, melt butter in a large frying pan over medium-high heat. Cook turkey, for 2 to 3 minutes on each side, or until browned and cooked through. Transfer to a plate and remove and discard toothpicks. Drain excess butter from pan, then return pan to heat.
5. Add Diane sauce to the pan and bring to boil. Return turkey steaks, then remove pan from heat.
6. Serve turkey with potato purée and charred broccolini.