Preheat oven to 450 degrees. Prepare one large baking dish (8"x13" or 9"x12") with cooking spray.
Trim four large chicken breasts, removing any fatty or undesirable parts. Pat the chicken dry and rub chicken on both sides with Greek Seasoning. Set aside.
Slice onion into large rings, drain olives and slice, rinse tomatoes, remove seeds from peperoncini and slice, juice 1/2 the lemon, slice the other 1/2, and crumble the Feta.
Place the onions, olives, tomatoes, and peperoncini on the bottom of the dish in an even layer. Lay the four chicken breasts evenly on top, then spread hummus on each chicken breast so that the entire breast is covered.
Sprinkle the crumbled feta over the top, and then drizzle the lemon juice over the chicken and vegetables. Then sprinkle the pan with sumac and place the lemon slices in between the chicken and vegetables if desired.
Bake for about 25-30 minutes, until the chicken is cooked through, the vegetables are tender, and the feta is starting to slightly brown. Sprinkle partially dried oregano over the top. Serve immediately over shredded cabbage.