Heat the oil in a very large skillet or a wok over medium-high. Add the onion and stir fry for 2 minutes. Add the garlic and stir fry for another 30 seconds, until you can smell the garlic. Turn the heat to high and add the chicken, stirring frequently for 5 or 6 minutes until browned and almost cooked throughout. Drizzle over the soy sauce and sesame oil, season with salt and pepper, and stir in the scallions, cooking for 2 more minutes, until the scallions soften and chicken is cooked through.
Remove from the heat and stir in the half of the lemon zest and all of the strained juice. Taste and add more zest as desired, and salt and pepper as needed. Serve over the hot rice in a shallow serving platter.