Preheat the oven to 200C/400F/Gas 6.
Place the potato, garlic, rosemary and sausages into a roasting tin and drizzle over the oil. Stir until well combined and coated in the oil, then season, to taste, with salt and freshly ground black pepper.
Transfer to the oven and roast for 12-15 minutes, until the sausages are cooked through and the potatoes are tender and golden-brow(Turn the mixture halfway through the cooking time to ensure even browning.)
Meanwhile, for the pesto, place all the ingredients into a food processor and blend until a smooth paste is formed.
To serve, spoon the sausages and potato onto a serving plate and drizzle the pesto around the edge of the plate.