In a large bowl, cover chickpeas with cold water and soak overnight.
Drain chickpeas and place in a large heavy saucepan; add onion, cover with water and bring to the boil over medium heat. Cook for 1 hour, or until chickpeas are very soft. Drain, reserving 250ml of cooking liquid. Allow to cool.
Meanwhile, in a large saucepan, cover the unpeeled beetroot with water and bring to the boil over medium heat. Cook until tender; drain and allow beetroot to cool before removing the skins and chopping.
Pure beetroot in a food processor; add the chickpeas, onions, tahini, garlic, lemon juice and cumin. Process until smooth. Slowly, while the machine is running, pour in the reserved cooking liquid and olive oil. Continue to process until mixture is thoroughly combined. Drizzle with a little olive oil to serve.