Preheat the oven to 425 degrees F. Place an oven rack in the middle position. Coat the inside of the cast iron Dutch oven (or skillet) with non-stick cooking spray. Set aside.
In a small bowl, mix together the salt, pepper, dried herbs, and garlic powder for the dry rub. Set aside.
Unwrap the game hens and use a paper towel to pat them dry. Rub the hens all over with olive oil and then rub the dry rub all over, inside and out, until coated. Place the game hens in the Dutch oven.
Arrange the onions, lemon slices, garlic, and fresh herbs around the hens.
Roast for 40-50 minutes or until cooked through. (The thickest part of the breast should have an internal temperature of 165 degrees F.)
Remove from the oven and let rest for 5-10 minutes. Serve the birds whole with the roasted onion and garlic along with your favorite side dishes. You can squeeze the roasted lemons over the hens for extra flavor if desired.
Notes
If you don't have a shallow Dutch oven or large cast iron skillet available, you could make the game hens in a roasting pan. This is also a good option if you want to double the recipe so there is room for everything in the pan.
Game hens are typically served whole, but if preferred, you can cut them in half down the center and serve half of a hen per serving. Place the hen halves skin side up for a nicer presentation.