1 pound boneless, skinless chicken breast halves, halved horizontally and thinly sliced
2 tablespoons olive oil
1 large onion, halved and thinly sliced
1 large red bell pepper, ribs and seeds removed, thinly sliced
3 garlic cloves, minced
1 and 1/2 teaspoons grated peeled fresh ginger
1/4 teaspoon red pepper flakes
3 tablespoons soy sauce
3 tablespoons rice vinegar
1 and 1/2 teaspoons cornstarch mixed with 1 tablespoon water
1 head iceberg lettuce, sliced in half to make "cups"
Season the chicken with salt and pepper. In a large nonstick skillet, heat 1 tablespoon of the olive oil over high heat. Add half the chicken; cook, stirring constantly, until opaque throughout, 2 to 4 minutes. Transfer to a plate. Repeat with remaining chicken.
Add the remaining tablespoon oil to the pan, along with the onion and bell pepper. Cook, stirring constantly, until the onion is tender and golden, about 5 minutes.
Reduce the heat to medium; add the garlic, ginger, and red pepper flakes to the pan; cook, stirring, until fragrant, 30 to 60 seconds.
Stir in the soy sauce, vinegar, and cornstarch mixture; remove from the heat. Add the chicken and any accumulated juices; toss to coat. Serve in lettuce cups.