Heat olive oil in a large skillet over medium heat. Stir-fry chicken until lightly browned and crisp.
In a small bowl, whisk together soy sauce, brown sugar, apple cider vinegar, mirin, and sesame oil. Set aside.
Add minced garlic to the center of the pan and sauté until fragrant (about 30 seconds). Pour in the sauce and cook, stirring, until it thickens into a shiny glaze (about 2-3 minutes).
If needed, whisk in the cornstarch mixture to thicken the sauce further.
Add the green beans and cook for 3-4 minutes until tender-crisp. Toss in sliced almonds.
Garnish with green onion slices and serve over steamed rice.