Drain the soaked chickpeas well and pat dry. Do not cook them. Add to a food processor with the onion, garlic, parsley, cilantro, cumin, coriander, and salt.
Pulse to a coarse, slightly grainy mixture that holds together when pressed. Add the baking soda and chickpea flour and pulse briefly to combine.
Chill the mixture for 30 minutes, then shape into 4 thick patties.
Heat a thin layer of oil in a large pan over medium heat. Fry the patties for 4 to 5 minutes per side, until deeply golden and crisp.
Whisk the tahini, lemon juice, minced garlic, and water into a smooth, pourable sauce.
Toast the buns, spread with tahini sauce, and stack with a patty, greens, pickled onion, and tomato. Serve right away.