Place the dried garbanzo beans in a large bowl and cover with at least 3 inches of water; cover and soak 20–24 hours until expanded (about 5 cups). Drain and rinse well.
Dry-roast the whole cumin and coriander seeds in a small heavy skillet over medium-high heat just until fragrant, being careful not to burn them. Let cool, then grind in a coffee or spice grinder or mortar and pestle.
Combine the soaked chickpeas, ground spices, chopped onion, minced garlic, minced preserved lemon peel, chopped parsley, chopped cilantro, lemon zest, salt, and pepper in a large food processor.
Pulse the mixture until finely ground but still slightly coarse — not a smooth paste. Avoid over-processing to prevent a crumbly falafel. Transfer mixture to the refrigerator and chill at least 1 hour.
Form the chilled mixture into balls about the size of golf balls, compressing firmly so they hold together; place on a plate while you heat the oil.
Fill a heavy skillet with 1½–2 inches of oil and heat to 360–375°F (use an instant-read thermometer). Test one falafel first to confirm the temperature.
Fry a few falafel at a time without overcrowding, turning carefully, until deep golden brown, about 2–3 minutes per side. Use a slotted spoon to transfer to a paper towel–lined plate to drain.
Serve falafel warm with hummus, preserved lemon cilantro yogurt sauce (or vegan yogurt sauce), pita, lettuce, tomatoes, onions, pickles, and a drizzle of tahini if desired.