Combine the olive oil, garlic powder, oregano, lemon juice, salt and pepper in a a bowl and pour into a freezer bag with the chicken marinade between 30 mins and 4 hours.
Combine the garlic clove, olive oil, vinegar, salt and pepper in a bowl. In another bowl after straining the greek yogurt, mix in the ranch dressing and feta. Then stir in the olive oil garlic mixture. Finally, add the cucumber.
Preheat the oven to 425 degrees.
Take the chicken out of the bag. Put on a baking sheet and put in the oven for 25-30 minutes After cooking cut the chicken into cubes and then put in a casserole pan. Mix in the tzatziki sauce and spinach once it is thoroughly mixed cover with aluminum foil and bake in the oven for 15 minutes. After that take the pan out , take off the foil and put the topping on the casserole and bake for another 5 minutes or until the topping is golden brown. Take out, let sit for 5-10 minutes then enjoy.