SOAK the chickpeas overnight in plenty of cold water, covered, on the kitchen bench (the dried chickpeas will swell to around 3 cups worth of soaked chickpeas). DRAIN the chickpeas well and place in a food processor. PULSE until they form a roughly chopped mixture, then add the onion, 1 clove of chopped garlic, parsley, cumin, coriander, 1 teaspoon of salt and olive oil and pulse more, scraping the sides occasionally, until the ingredients are well combined and the mixture holds together. HEAT 3 tablespoons of olive oil in a large frying pan over medium heat. FORM golf ball-size spoonfuls of the mixture (about 1 heaped tablespoon each) and flatten slightly with the palm of your hand (dampen your hands first to help prevent sticking). FRY half the patties (about 9) for 4 minutes on each side until crisp and golden. REPEAT the process with the remaining olive oil and falafel mixture. TO make the tahini sauce, place the tahini, 2 minced garlic cloves, ½ teaspoon salt, lemon juice and water into a small bowl and mix thoroughly to combine.