This sandwich is also delicious cold the next day for lunch—to prevent the pita from getting soggy, don't assemble until you're ready to eat. We like thick, tangy Greek-style yogurt in this dish.
Place eggplant slices in a colander; sprinkle with 1 tablespoon salt. Toss well. Drain 30 minutes. Rinse thoroughly; pat dry with paper towels.
Combine remaining 1/2 teaspoon salt, yogurt, and next 4 ingredients (through garlic) in a small bowl.
Preheat grill to medium-high heat.
Brush eggplant and onion slices with oil. Place eggplant and onion slices on grill rack coated with cooking spray; grill 5 minutes on each side or until vegetables are tender and lightly browned.
Fill each pita half with 1 1/2 tablespoons yogurt mixture, one quarter of eggplant slices, one quarter of onion slices, and 1/4 cup arugula.
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