Put a baking sheet on the middle oven rack and preheat to 450 degrees F. Pulse 2 cups cilantro, 1 cup onion, the garlic, cumin, 1/2 teaspoon salt and a few grinds of pepper in a food processor until finely chopped. Add the chickpeas and 2 tablespoons tahini; pulse to make a chunky paste. Scrape the mixture into a bowl and stir in the panko.
Form the chickpea mixture into twelve 2-inch patties (about 2 tablespoons each). Remove the baking sheet from the oven, brush with 1 tablespoon olive oil and add the chickpea patties. Bake, flipping the patties and rotating the pan halfway through, until golden and crisp, 20 to 25 minutes.
Meanwhile, mix the remaining 1/3 cup tahini with 6 tablespoons warm water and the juice of 1 lemon in a small bowl until creamy (it may look separated at first but it will come together). Season with salt and pepper; set aside. Toss the cucumbers with the juice of the remaining lemon, 1 cup cilantro, 1/2 cup sliced onion and 1 tablespoon olive oil in a medium bowl; season with salt and pepper.
Warm the pita in the oven. Serve the falafel with the cucumber salad, pita and tahini sauce.