To prepare this Greek chickpea soup recipe (revithia soupa), start by washing the chickpeas and let them soak in plenty of cold water overnight (ideally for 18 hours). When you are ready to make this Greek chickpea soup recipe, drain the chickpeas (revithia) and discard the soaking water. Place the chickpeas in a colander and rinse very well with plenty of water (4-5 times). Put your hands in the colander and rub them well. Place the revithia in a large pan with enough cold water to cover them (about 4cm over the chickpeas). Turn the heat up and bring to the boil. Keep removing the froth from the top with a wooden spoon for about 15 minutes. Drain the chickpeas and then put them back in the pan and add enough warm water to cover them again. Bring to the boil and add the olive oil, chopped onion, oregano and bay leafs. Turn the heat down to a simmer and place the lid on. Let the revithia cook for 1-2 hours, depending on the chickpeas, until tender. Add more water if necessary in order not to dry. If you prefer your soup to be thicker, during the end of cooking time add in a bowl the flour and lemon juice and pour gradually 1-2 ladles of the soup and whisk. Add the mixture slowly in the pan with the chickpeas and season. Stir gently for 2-3 minutes until the soup thickens. Serve this Greek chickpea soup while still warm with freshly ground pepper and lots of olives and crusted bread aside. For extra aroma and taste, just add some drops of olive oil on top of soup.