Put the chickpeas, onion, 2 garlic cloves, cilantro, cumin, salt and chili flakes in the bowl of a food processor and pulse until combined but still chunky, scraping down the sides of the bowl once or twice.
Add the flour and baking powder and pulse until you have a soft mixture that you can roll into balls. Roll into meatball-sized balls, and flatten each slightly.
Stir together the yogurt, lemon juice and a big pinch of salt.
Finely crush or grate and stir in the last garlic clove.
In a shallow pot or skillet, heat about 1/2-inch of oil until it’s hot but not smoking.
Cook the falafel for a few minutes per side, without crowding the pan, until they’re golden and crisp.
Transfer to a paper towel lined plate.
Serve immediately, wrapped in lettuce leaves, with garlic sauce and Sriracha.