Preheat the oven to 180°C, fan 160°C, gas 4. Place the whole garlic bulbs on a baking tray and roast for 35-40 minutes.
In a food processor, whizz the chickpeas, tahini, 50ml olive oil, 2 tablespoons warm water and lemon juice together until smooth, adding another tablespoon of warm water, to loosen a little if necessary.
Unpeel the roasted garlic and squeeze the garlic pulp into the food processor. Whizz again and season to taste. Serve drizzled with extra olive oil and with flatbread.