Reduce heat under skillet to low. Add half of one medium yellow onion to skillet and stir to incorporate. Sauté onions 3 to 5 minutes or until translucent.
Add 2 cloves garlic to skillet with sautéed onions and stir to incorporate. Sauté garlic 30 seconds or until just fragrant.
When garlic is fragrant, add 2 cups low-sodium chicken broth to skillet and stir to incorporate, making sure to scrape up any browned bits of food that may have stuck to bottom of pan.
Add 1 ½ cups dry orzo and ½ cup sliced kalamata olives to skillet. Stir to incorporate.
Return chicken thighs to skillet, allowing chicken to settle into orzo mixture. Cover skillet with lid and increase heat to medium-high.
Once broth begins to boil rapidly, immediately reduce heat under skillet to low. Simmer chicken and orzo mixture 15 minutes or until majority of chicken broth has been absorbed into orzo.
After 15 minutes, or once most of broth is absorbed, uncover skillet and set lid aside. Check chicken thighs with internal meat thermometer. If chicken thighs register between 165° to 170° Fahrenheit, transfer chicken thighs to large plate and set aside. Note: If chicken thighs register below 165° Fahrenheit, recover skillet and cook chicken another 3 to 5 minutes, then check temperature again and repeat as needed. Once chicken reaches target temperature range, transfer thighs to large plate and set aside.
Taste orzo. If orzo is not tender, return lid to skillet and continue cooking orzo until tender, stirring occasionally. If needed, stir in additional ¼ cup low-sodium chicken broth in small increments.
When orzo is tender, add 2 cups fresh spinach leaves to skillet and stir until spinach is fully incorporated into orzo mixture.
Add ½ cup crumbled feta, salt (to taste), and freshly cracked black pepper (to taste). Gently fold in ingredients, making sure all broth has been fully absorbed.