Preheat oven to 375 F.
In a large skillet, add olive oil and heat to medium/high heat.
Season chicken breast on the tops only with Za'atar spice blend, salt, black pepper and dried oregano. We are only seasoning the tops because we don't want to scorch the herbs.
Place chicken breast in the skillet, unseasoned side down.
Cook for 8-10 minutes, reducing heat if the chicken starts to over brown.
Flip chicken over and cook for another 5 minutes.
Add butter, red onion and squeeze lemon over onions and chicken.
Sauté onions around the chicken for about 5 additional minutes, adjust the heat if onions start to over cook.
Flip chicken back over to the spiced side, pile the red onions on the tops of the chicken.
Place in oven and cook for an additional 10-15 minutes or until internal temperature is 165 F.. The time will vary depending on the thickness of your chicken breasts. If they are thin you may only need 5 minutes in the oven, if they are thick you may need 20 minutes.
*if you don't have an oven proof skillet or don't want to transfer this into an oven you can finish cooking on stove top until internal temperature is 165 F.
Remove chicken from oven with an oven mitt, it's a totally common mistake to grab the handle without one so be careful.
Once removed, sprinkle feta crumbles or cubes over chicken.
Let chicken rest for 5 minutes so juices can recirculate and enjoy.