For the falafel: Combine the chickpeas, parsley, cilantro, onion or scallions, garlic, cumin, ground coriander, salt, cornstarch and baking soda in a food processor and pulse until finely chopped and well combined. Transfer the mixture to a large bowl.
Heat several inches of vegetable oil in a large, heavy pot or Dutch oven to 400 degrees F.
Meanwhile, spread the sesame seeds on a rimmed baking sheet. Take about 1 tablespoon of the chickpea mixture, roll it into a ball and gently press into the sesame seeds, flattening slightly. Repeat with the remaining mixture.
Fry the balls in batches until golden and crispy, 5 to 7 minutes. Use a slotted spoon to remove to a paper towel-lined plate and keep warm. Repeat with the remaining falafel balls. (Be sure to let the oil return to temperature between batches.)
For the tahini sauce: Add the tahini, water, lemon juice, cumin and salt to a small food processor or blender. Blend the ingredients until smooth and adjust the seasoning as needed.
Open the pita bread and spread with the tahini sauce. Add the falafel, crushing as you add it to the bread. Add the pickles, tomato slices and arugula.