Preheat oven to 200°C (180°C fan-forced). Line a large rimmed baking tray with baking paper.
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.
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.
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.
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.
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).
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.