Preheat your oven to 200 degrees celsius.
Cut both ends from your leek. Because we're effectively roasting the leek, we can use both the green and white parts. Make sure to rinse the leek really well (I slice it long ways and then rinse under running water). Slice into 1cm wide slices.
In the bottom of a baking pan (or casserole dish), drizzle a couple of tbsps of olive oil. Scatter the leek over the oil.
Lay your sausages over the leek. Drizzle with olive oil, and season with salt and pepper.
Pop into your oven, and cook for approx. 45mins. You will need to check these and turn the sausages over at least once, half way through. The cooking time will depend on the thickness and size of your sausages - so keep an eye on them.
Serve with mashed potato, and steamed vegetables.