Combine the lime juice and tuna in a medium bowl. Combine teriyaki sauce and cornstarch in a small bowl, stirring with a whisk.
Heat the oil in a large nonstick skillet over medium-high heat. Add the onion, ginger, and garlic; stir-fry for 2 minutes. Add tuna mixture; stir-fry for 2 minutes. Add teriyaki mixture; stir-fry 30 seconds or until slightly thick. Serve over rice, and sprinkle with cilantro.