Preheat the oven to 180 C degrees.
Cut the potatoes into chunks and put them into a pan of cold water. Bring to the boil and simmer for 10 minutes, then drain.
In the meantime, peel & cut the sweet potatoes into bite-size chunks, slice the pepper and cut the onion into wedges.
Scatter these around in a large roasting tin. Add the potatoes. Drizzle some olive oil over the vegetables then season with salt and pepper. Scatter the rosemary leaves over. Toss everything well so that the vegetables are covered with oil.
Put the sausages between the vegetables and put the tin into the oven.
Bake for about 25-30 minutes, until the vegetables are roasted and the sausages are brown, turning them over a couple of times during cooking.
Serve warm.