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Heat oven to 180C/160C fan/gas 4. Top and tail the beetroot, then peel and cut into wedges. Put the beetroot in a roasting tin, drizzle over the olive oil and toss with the onion, garlic and caraway seeds. Roast for 45 mins until the beetroot is tender and the onion is sticky and charred.
Heat the grill and cook the sausages until golden. Make the dressing by whisking together all the ingredients. While the beetroot is still warm, toss with the lentils and dressing. Add the watercress and serve with the sausages.