The night before, put the chickpeas in a bowl with plenty of water (the water should cover them well because they will expand) and the baking soda. The next day, drain them and spread them out on a clean towel or paper towel to dry.
Transfer the chickpeas to the bowl of a food processor fitted with a metal blade. Add the onion, garlic, coriander, parsley, mint, spring onion, cumin, dry coriander, black pepper, salt and tahini. Grind until the ingredients are very, very finely chopped. Press some of the mixture into your fingers. If it comes together and doesn't fall apart, it's ready.
Take about 1 tablespoon of the mixture and make small balls with your hands. Press the balls a little with your palm so that they are not completely spherical.
Put about 1 cm of vegetable oil in a frying pan over medium-high heat. Fry the falafels for a few minutes, turning them 1-2 times, until they turn a dark bronze colour and become crispy. Remove them to a plate lined with kitchen paper to remove excess oil. Serve immediately with tahini sauce.