For the Orange and Rosemary Purée, juice the carrots and oranges through an electric juicer. Place the juice into a medium saucepan saucepan over high heat and bring to the boil. Reduce heat to medium, add the orange peel and rosemary sprigs and cook until the skins are tender, about 15 minutes.
Remove from the heat. Use a stick blender to purée until smooth. Add the vinegar and sugar and stir to combine. Season with salt, to taste and set aside.
For the Pickled and Roasted Mushrooms, place the mushrooms into a bowl and set aside.
Combine the sugar, vinegar and rosemary with 400ml of water in a small saucepan over medium heat and bring to the boil. Remove from the heat, pour the pickling liquid over the mushrooms and set aside to pickle for 30 minutes. Drain the mushrooms.
Heat a large frypan over medium heat. Add the drained mushrooms and cook, turning, until golden and caramelised. Remove from the frypan and set aside.
For the Lamb, remove the fillets from the lamb racks. Trim off all the fat and sinew from the fillets. Reserve the bones and trimmings for later use.
Wrap each fillet tightly in cling film into a cylindrical shape and tie a knot at each end to secure. Set fillets aside in the fridge.
For the Sauce, chop the reserved lamb rack bones and trimmings into small pieces. Place a large frypan over high heat. Add the chopped bones and trimmings and fry, turning occasionally, until golden brown, about 5-6 minutes. Add the chicken stock, veal stock, and reserved carrot and mushroom trimmings and reduce heat to medium. Simmer for 40 minutes then remove from the heat and strain through a fine sieve into a clean saucepan.
Place the saucepan over high heat and reduce the liquid to a sauce consistency, about 7-8 minutes. Season to taste then transfer the sauce to a serving jug, keeping warm.
For the Rosemary Oil, place the rosemary leaves into a small saucepan. Cut the top third from the bunch of chives and add to the saucepan of rosemary leaves. Cover with the grapeseed oil then place over low heat and bring to 60°C.
Once at temperature, move the saucepan on and off the heat to maintain the temperature for 9 minutes. Remove from the heat and strain through a fine sieve into a jug. Set aside to cool.
For the Leek and Rosemary Ash, peel the outer leaves from the leek. Set the peeled leek aside for later use. Spread the outer leek leaves and rosemary leaves onto a baking tray lined with baking paper. Place into the oven to dry, about 10-12 minutes. Once dry, use a blow torch to char the leaves and set aside to cool. Push the charred leaves through a fine sieve onto a plate and reserve.
For the Roasted Leek and Onion Hearts, cut the leek and onion in half. Place a large frypan over medium heat. Add the grapeseed oil and heat then add the onion and leek, cut side down, and cook until caramelised, about 7-8 minutes. Remove from the heat, season with salt and set aside to cool. Once cool, separate the leek and onion into layers then set aside.
To finish the Lamb, heat a large frypan over medium heat. Unwrap the lamb fillets and coat with oil, salt and pepper. Add to the frypan and cook, turning halfway, until medium rare, about 8-9 minutes. Remove the fillets from the pan and roll in the Leek and Rosemary Ash. Set aside to rest.
To serve, divide the Pickled Roasted Mushrooms between serving plates and top with half of a Lamb fillet. Add some Roasted Leek and Onion Hearts, dot with Orange and Rosemary Purée and serve the Sauce on the side.