Combine orange juice, lime juice, garlic, 1/2 tsp salt a a pinch of pepper in a blender until combined. Let sit for 5 minutes. Turn blender back on and slowly pour 1/2 cup of olive oil.
Remove 1/2 cup of the mixture and set aside for the marinade
Add parsley to the remaining mixture in the blender and puree until smooth. Season with salt and pepper. Transfer the sauce to a bowl, cover and refrigerate
Season the chicken with salt and pepper. Place in a bowl and add the reserved marinade, cumin and oregano. Rub util evenly coated. Cover and refrigerate 1 to 4 hours.
Preheat over to 450 degrees. Let the chicken come to room temperature. Toss plantains and onions on a baking sheet with the remaining 2 tbsps of olive oil. Season with salt and pepper. Spread out on the baking sheet. Nestle the chicken, skin side-up, and orange slices among the plantains and onions.
Roast until the plantains are tender, about 20 minutes. Remove the plantains to a cutting board and stand them on a cut side. Flatten them with the bottom of a ramekin, and return them to the baking sheet, setting them flat in the pan juices. You may need to add extra olive oil on top of the plantains if they look dry.
Return the pan to the oven and roast until the chicken is browned, 20 to 25 more minutes. Sprinkle with chopped parsley, server with the sauce on the side.