Preheat the oven to 180C/350F/Gas 4.
Heat an ovenproof frying pan and add the carrots and butter and squeeze in the orange juice. Cook until the butter is melted and the juice is sizzling.
Add the squeezed orange halves to the pan, cut-side down, then season with salt and freshly ground black pepper and transfer to the oven. Roast for 8-10 minutes, spooning the liquid over the carrots occasionally until the carrots are tender and the liquid has turned into a sticky glaze.
To serve, pile the carrots into a serving dish and garnish with the orange halves.