The day before the meal, combine brine ingredients in a large plastic bag. Add chickens and refrigerate for 24 hours.
Preheat oven to 500°F.
Boil water in a medium pot and cook onions 3-4 minutes. Drain and cool in ice bath. Squish onions away from their skins, leaving them whole. Place in a shallow roasting dish or casserole. Add potatoes, garlic, lemon slices, olives, bay, rosemary and thyme. Drizzle with EVOO; season with salt and pepper.
Place a cooling rack over baking dish.