First, make the pickled red onions, bring the water to a boil in a small saucepan. In small bowl, add the vinegar, sugar, salt, and peppercorns. Add the boiling water to the vinegar mixture. Stir until the salt and sugar have dissolved. Submerge the sliced onion in the pickling solution and let sit for 30 minutes. You can make the pickled onions in advance and keep covered in the refrigerator for up to 3 weeks.
To make the falafel, preheat the oven to 425 degrees F and line a large baking sheet with parchment paper. Rinse the chickpeas in a strainer, shaking off as much water as possible. Set aside.
In a high powered blender or food processor, blend the oats until they are a fine powder, flour consistency. Pour into a medium bowl. Stir in the cumin, coriander, salt, baking soda, and cayenne, and set aside. In the blender, combine the cilantro, parsley, onion, and garlic. Pulse to coarsely chop. Add the chickpeas and pulse on low until a chunky mixture forms (not as smooth as hummus), stopping to scrape down the sides every so often. Add the chickpea mixture to the oat mixture in the bowl, and stir until evenly combined. Using a 2 teaspoon-sized spring release scoop, scoop two balls and shape into a 2-inch patty, making about 18-20 patties. Place on the prepared baking sheet. Just before placing in the oven, liberally spray both sides of the patties with cooking spray. Bake for 12 minutes on each side.
While the falafel is baking, make the tahini sauce. Whisk together all of the sauce ingredients until smooth.
To assemble the pitas, place the pitas directly over a gas flame to char the edges. Cover to steam. You can also heat the pitas in a pan on the stovetop, cover to steam. Layer each pita evenly with greens, 3 to 4 falafel, quick-pickled onions, and a drizzle of tahini sauce. Serve immediately.
Notes-Uncooked, shaped patties freeze well. To freeze, place the patties on a baking sheet in the freezer for about 30 minutes or until hardened. Store in a freezer-safe ziplock bag with as much air removed as possible for up to 2 months. Before baking, spray patties liberally with cooking spray. Bake at 425 degress F for 15 minutes on each side. The tahini sauce will keep in the fridge for up to 2 weeks and the onions will keep covered in the fridge for 3 weeks.