Drain chickpeas and place in a large saucepan, along with lamb bones, if using. Add 1 onion, halved, 3 garlic cloves and fill saucepan with enough water to come about 4cm above bones, then place over a high heat. (Do not add salt, it will toughen chickpeas.) Bring to a boil and cook for 5 minutes, skimming off any fat, then reduce heat to low and cook, covered, for 1 hour 20 minutes or until chickpeas are tender. Meanwhile, roughly chop remaining onion and garlic. Heat 3 tablespoons of oil in a large frypan over a medium heat. Add onion and garlic, and cook, stirring constantly, for 3 minutes. Stir in 2 teaspoons of ground cumin, season with sea salt and ground black pepper, and cook, stirring constantly, for 5 minutes or until onion softens. When chickpeas are cooked, discard onion and bones, add onion mixture and half the lemon juice to saucepan, and stir to combine. Next, blitz half the chickpea mixture in a food processor until smooth, set aside, then barely blend remainder, so it remains chunky. Return both batches to saucepan and cook over a medium heat for 10 minutes, or until soup is very hot (do not bring to a boil). Next, stir in a handful of mint, 2 tablespoons of olive oil and season well with ground black pepper. Taste and add more sea salt and lemon juice as required. Ladle into bowls, top with mint leaves, drizzle with olive oil and sprinkle over cumin.