Step 1
Preheat oven to 200°C (180°C fan-forced). Line a large rimmed baking tray with baking paper.
Step 2
To make the balsamic brown butter, in a large frying pan over medium heat, cook the butter, stirring often, for 3 mins or until the foam has subsided and the butter is a deep golden brown. Transfer the butter to a heatproof bowl and stir in vinegar and honey. Season with salt and pepper and cover to keep warm.
Step 3
Coat pumpkin with oil and season with salt. Place pumpkin, cut-side down, on the lined tray. Roast for 20 mins, then set aside to cool slightly (par-cooking makes the pumpkin easier to cut). Using a thin metal spatula, transfer pumpkin to a cutting board. Using a sharp knife, cut slits into pumpkin crossways about 5mm apart, cutting as deep as you can without slicing all the way through. Return the pumpkin to the lined tray.
Step 4
Meanwhile, heat a large frying pan over medium-high heat. Coat the lamb with oil and sprinkle with salt and pepper. Add the lamb and cook, turning, for 10-12 mins or until brown all over.
Step 5
Arrange lamb next to the pumpkin on the tray. Using a pastry brush, coat lamb with some balsamic brown butter and sprinkle with chopped rosemary.
Step 6
Roast lamb and pumpkin, brushing pumpkin every 15 mins with balsamic brown butter, for 1 hour or until an instant-read meat thermometer inserted in the centre of the lamb registers 60°C and the pumpkin is tender and glazed. Transfer lamb to a carving board and rest for 15 mins (if lamb is ready before pumpkin, return the pumpkin to oven to finish cooking while the lamb rests).
Step 7
Reheat the remaining balsamic brown butter over low heat. Arrange the lamb and pumpkin on a serving platter. Serve with the rocket, rosemary sprigs and remaining balsamic brown butter.