Season chicken with salt & pepper. In a large nonstick skillet, heat 1 tbsp of oil over high heat then add chicken; cook, stirring constantly, until cooked throughout. Transfer to a plate.
Add remaining tbsp of oil to pan. Add onion and bell pepper. Cook, stirring constantly, until onion is tender and golden, about 4 minutes (reduce heat if browning too quickly).
Reduce heat to medium; add garlic and red-pepper flakes about 1 minute.
Stir in soy sauce, vinegar, and water; remove from heat. Add chicken and any accumulated juices; toss to coat. Serve in lettuce.
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