Preheat a large skillet over medium-high heat and add olive oil, allowing it to heat until shimmering.

Season chicken breasts with salt and pepper, then sear them in the hot skillet for 5-7 minutes per side until fully cooked.

In the same skillet, add more olive oil and sauté diced red onion, minced garlic, zucchini, and red bell peppers for 4-5 minutes until tender.

Return the cooked chicken to the skillet with the sautéed vegetables and stir well for another 2-3 minutes.

Drizzle lemon juice over the stir fry and add olives and feta if using, heating for another minute before serving.