In a small bowl, whisk the lemon juice with the honey and half of the lemon zest. Set this bright glaze aside.
Heat a large wok or skillet over high heat. Add 1 tablespoon of olive oil, then the green beans and a pinch of the salt. Stir-fry 3 minutes until lightly blistered and bright green. Splash in a tablespoon of water, cover for 1 minute to steam, then transfer beans to a plate.
Pat the chicken dry. In a bowl, toss it with cornstarch, the remaining salt, black pepper, and red pepper flakes until the pieces are evenly coated.
Return the wok to medium-high heat and add 2 tablespoons olive oil. Spread the chicken in a single layer and cook 2 minutes undisturbed, then stir-fry 3–4 more minutes until golden and cooked through. Transfer to the plate with the beans.
Reduce heat to medium. Add the red onion to the pan (add a teaspoon more oil if the pan looks dry) and cook 2 minutes until just softened. Stir in the garlic and cook 30 seconds until fragrant.
Pour in the lemon-honey mixture and the remaining lemon zest. Let it bubble for 60–90 seconds, scraping up any browned bits, until slightly thickened and glossy.
Return the chicken and green beans to the pan and toss 1–2 minutes to coat in the sauce. Taste and adjust seasoning with a pinch more salt or a squeeze of lemon if desired. Serve hot (great over steamed rice).