Tzatziki
Bring the water to a boil. Add the quinoa, reduce the heat to low, cover, and simmer for 15 to 20 minutes until all the liquid is absorbed. Set aside.
Preheat the oven to 375°F.
Pulse together the chickpeas, onion, garlic, parsley, salt, pepper, and other spices in a food processor.
Process until everything is finely chopped. It won't become completely smooth, and that's okay.
Transfer to a bowl and mix in the quinoa. Add the egg yolk and stir until fully incorporated.
Shape the dough into eight balls, and then flatten with your palm into about ½-inch-thick patties.
Transfer to a parchment-lined cookie sheet.
Bake for 10 minutes. Remove from the oven, flip each patty, and bake for another 10 minutes.
Heat half of the oil in a large skillet over medium-high heat. Place four of the patties in the hot skillet and fry for 2 to 3 minutes on each side until golden brown. Repeat with the remaining oil and remaining patties.
Tzatziki
Place the grated cucumber in a fine-mesh strainer set over the sink or a bowl, and let it drain for 10 to 15 minutes.
Combine all the ingredients in a single bowl, and mix well. Taste and adjust the seasoning to your liking.
Fill each pita half with two falafel patties, tzatziki, and any other toppings you like (recommend crunchy romaine leaves, tomato slices, and red onion).
