Place chickpeas, tahini, egg, garlic, 3 teaspoons of the Za’atar Seasoning and salt in food processor; cover. Process until almost smooth, scraping down sides as necessary. Transfer to large bowl. Add flour, onion and baking powder; mix well. Form mixture into 12 patties, each about 1/2-inch thick.
Pour enough oil into large skillet to come 1/4-inch up sides of pan. Heat on medium-high heat until shimmering. Cook patties in batches 3 minutes per side or until golden brown. Drain on paper towels.
Mix yogurt, cucumber, lemon juice and remaining 2 teaspoons Za’atar Seasoning in medium bowl until well blended. To serve, place 1 falafel into each pita pocket. Add 1/4 cup spinach and 1 tablespoon each feta cheese and red onion. Drizzle with about 2 tablespoons of the yogurt mixture before serving.