In a small bowl, whisk together pomegranate molasses, oil, and 1/2 teaspoon pepper. Place each hen in a large resealable plastic bag. Divide marinade, sliced onions, and garlic evenly among bags. Seal bags; turn to coat. Refrigerate at least 4 hours (or up to overnight), turning occasionally.
Preheat oven to 350 degrees, with rack in lower third. Set roasting racks in 2 shallow pans or on 2 rimmed baking sheets, and coat racks with cooking spray.
Remove hens from plastic bags, letting excess marinade drip off. Place hens on racks, breasts side up, and tuck wing tips under. Fill cavity of each hen with 4 onion and 4 lemon wedges; tie legs with kitchen twine. Brush all over with butter; season with salt and pepper.
Transfer pans to oven, and roast until hens are golden and cooked through, about 1 1/2 hours, rotating pan every 30 minutes. (If skin starts to brown too quickly, tent loosely with foil.) Serve warm.