In a small bowl whisk together the marinade ingredients: olive oil, lemon juice, garlic, oregano, kosher salt and pepper.
In a or bowl sealable plastic bag, add the chicken and marinade. Toss to coat or squeeze air out of bag and seal. Massage chicken with marinade and refrigerate 4 hours or overnight.
Preheat oven to 400°F.
Place the spinach in a rectangle 9x12" casserole dish that has been sprayed with cooking spray or olive oil and sprinkle with the red pepper flakes.
Layer the chicken breasts on top of the spinach. Sprinkle the artichoke hearts, baby tomatoes, black olives and marinade on top of and around the chicken.
Bake covered for 25 minutes, depending on the thickness of your chicken breasts. (See Note 1)
Remove from oven and add feta cheese crumbles to each piece of chicken. Put it back in the oven, uncovered for 5 minutes. Allow to rest 5 minutes before slicing or serving.
Video
Notes
If chicken breasts are too thick, slice in half horizontally and pound between sheets of plastic wrap for even thickness. For best results, use an instant read thermometer, the chicken is cooked when it reaches internal temperature of 165°F.