Begin by slicing the chicken, zucchini, bell pepper, and onion to ensure a smooth cooking process.

In a large skillet or wok, heat the vegetable oil over medium-high heat until it shimmers.

Add the sliced chicken to the pan and season with a pinch of salt and pepper. Stir-fry until golden brown and cooked through, about 5-7 minutes, then remove from the pan.

In the same pan, add the onion and garlic and cook for about 2 minutes until fragrant. Then toss in the zucchini and bell pepper, stir-frying until tender yet crisp.

Return the cooked chicken to the skillet, pour in the soy sauce, and stir everything together for an additional 2 minutes to meld the flavors.

Remove from heat and transfer to a serving dish. Garnish with optional sesame seeds and chopped green onions.