Combine the chickpeas, onions, parsley, lemon juice, garlic, coriander, cumin, salt, turmeric, and cayenne in the bowl of a food processor. Process on high speed until it forms a smooth paste, scraping the sides of the bowl as needed. Transfer to a bowl and add the baking powder and flour. Adjust the seasoning, to taste. Refrigerate until firm, about 20 minutes.
Heat the oil to 360 degrees F. in a deep pot over medium-high heat.
With damp hands, shape the chickpea batter into balls about 1 1/2 tablespoons each and then flatten into patties. Fry in batches until golden brown, about 3 minutes. Drain on paper towels.
Open the pita bread halves and insert 2 or 3 falafel patties into each. Drizzle with the tahini sauce and layer the tomatoes and lettuce on top. Serve immediately.
Yield: about 8 servings