Preheat the oven to 400°F. Pat the hens dry with paper towels, inside and out.
Rub the hens all over with the oil. Mix the salt, pepper, garlic powder, thyme and rosemary and sprinkle the seasoning evenly over both hens.
Tuck half a lemon and 2 garlic cloves into each cavity, along with any fresh herbs.
Place the hens breast side up on a rimmed baking sheet or in a roasting pan, a few inches apart.
Roast for 50–60 minutes, until the skin is golden and a thermometer in the thickest part of the thigh reads 165°F.
Rest for 10 minutes before serving whole or cutting in half.