Make the falafel:
Preheat the oven to 400°F. Place the oil on a small rimmed baking sheet and place in the oven while it preheats. Combine the chickpeas, onion, flour, garlic, parsley, cilantro, sesame seeds, salt, cumin, coriander, pepper, and chili flakes in a food processor and process until the mixture looks like fine crumbs, 20 to 25 pulses. Pinch a bit of the mixture together between your fingers; if it holds together you’re good. If it crumbles, add another tablespoon of flour to the processor and pulse 5 more times. Form the mixture into four equal-sized patties about 3 1/2 inches wide by 3/4 inch thick. At this point your oil should be hot; open the oven and working quickly, place the baking sheet on the range and gently arrange the patties on the sheet; they should sizzle on contact. Return to the oven and bake until the underside is crisp, 13 minutes. Open the oven and, using a spatula, gently flip the patties and bake until the second side is crisp, another 12 to 13 minutes. While the falafel is baking, make the salad and tahini:Combine the tomatoes and cucumbers in a bowl and season with olive oil, lemon juice, salt, and pepper to taste. Stir in the parsley.
Make the tahini sauce:
Whisk together the tahini, water, lemon juice, and in a small bowl until smooth.
Build the sandwiches:
When the falafels are ready, take them out and put the pitas right in the oven to warm for 2 to 3 minutes. Make sandwiches by stuffing a lettuce leaf and some salad into a warm pita, followed by a falafel patty, and some pickles and red onion slices. Drizzle with the tahini sauce.
