Position racks in the upper and lower thirds of the oven and preheat to 475 degrees F. Cut the hens in half with kitchen shears, removing the backbone and cutting through the middle of the breastbone. Divide the onion, oranges and rosemary sprigs between 2 rimmed baking sheets. Arrange the hens skin-side up on the baking sheets and season all over with salt and pepper. Set aside at room temperature, 30 minutes.
Meanwhile, combine the orange juice, cranberries, maple syrup, 1 teaspoon coriander, the chile powder and 1/4 teaspoon salt in a medium saucepan. Bring to a boil, then reduce the heat to a simmer. Cook, mashing the cranberries as they start to burst, until the mixture is thickened and reduced to about 3/4 cup, 20 to 25 minutes. Stir in the vinegar and season with salt. Set aside.
Melt the butter in a small saucepan over medium heat. Reduce the heat to low and add the chopped rosemary, garlic and the remaining 2 teaspoons coriander. Cook 30 seconds, then remove from the heat.
Pat the hens dry with paper towels, then brush with the flavored butter. Roast the hens until the skin is browned and a thermometer inserted into the thigh registers 165 degrees F, 25 to 30 minutes. Remove from the oven and switch to the broiler setting. Brush the hens with the orange-cranberry sauce, then broil, one baking sheet at a time, until glazed and dark brown in spots, 2 to 3 minutes.