Thinly slice the chicken, red bell pepper, and red onion; pit and halve the Kalamata olives. Whisk together lemon juice, red wine vinegar, dried oregano, garlic powder, salt, black pepper, and chicken broth for the sauce, then set aside.

Heat a large skillet or wok over high heat with olive oil until shimmering. Add thinly sliced chicken in a single layer and cook undisturbed for 1-2 minutes until golden brown on one side, then stir and cook for another 2-3 minutes until cooked through. Remove the chicken and set aside.

Add the sliced red bell pepper and red onion to the same pan and stir-fry for 2-3 minutes until they begin to soften but retain a slight crunch.

Return the cooked chicken to the pan with the vegetables, add the chopped spinach and Kalamata olives, then pour the whisked sauce over everything.

Toss everything together quickly for 1 minute to wilt the spinach and coat all ingredients with the sauce. Remove from heat, garnish with crumbled feta cheese, and serve hot.