Drain and rinse the chickpeas, then pat them very dry with a paper towel to ensure a crispy texture.
In a food processor, combine the chickpeas, egg, parsley, cilantro, garlic, cumin, coriander, salt, and pepper.
Pulse the mixture until well combined but still slightly textured; do not over-process into a smooth paste.
Form the mixture into 4 to 5 small patties, about 1/2 inch thick.
Heat the olive oil in a non-stick skillet over medium-high heat.
Place the patties in the skillet and cook for 3-4 minutes per side until deeply golden brown and crisp.
While the falafel cooks, whisk together the Greek yogurt, tahini, lemon juice, and water in a small bowl until smooth.
Serve the warm falafel patties immediately, drizzled generously with the lemon-tahini sauce.