Cut chicken by the backbone and remove the backbone, keeping the rest of the chicken in one piece. Place the chicken in a sealable plastic bag or a baking dish large enough to hold the chicken.
In a blender, combine 1/2 the garlic, oregano, cumin, 1/2 cup lime juice, Worcestershire sauce, sherry and 1/4 cup olive oil. Puree until smooth. Pour over the chicken and make sure the chicken in entirely coated. Marinate, refrigerated, overnight.
Preheat a grill. Preheat the oven to 350 degrees F.
Grill the chicken over medium heat, using something heavy to weigh the chicken down, for 5 minutes per side. Place in an ovenproof dish or saute pan and finish cooking in the oven for 12 minutes.
In a small saucepan, heat the remaining olive oil. Add the remaining garlic and onions, and cook until fragrant. Let cool and add the remaining lime juice, orange juice, and parsley. Adjust seasonings and serve with the grilled chicken.