Preheat the oven to 350°F/180°C.
In a small bowl, combine all of the dry ingredients and set aside.
Prep the Cornish hens: pat dry with kitchen towels, then stuff each Cornish hen cavity with a piece of onion, garlic, lemon, and a sprig of rosemary. Truss the legs with butcher twine and tuck the wings behind the bird.
Drizzle with half of the olive oil, then season thoroughly with the dry rub.
In a 9x13-inch roasting dish, add the onion, garlic cloves, lemon, and rosemary sprigs. Place the Cornish hens on top of the aromatics, and drizzle with the rest of the olive oil.
Add ⅓ a cup of water or chicken stock to the pan.
Roast uncovered in the oven for 1 hour or until the internal temperature reaches 180°F/83°C.
Remove the Cornish Hens to a plate, and tent with foil to rest. Pour the juices into a small saucepan, and reduce for 5 minutes to create a sauce.
Cut each Cornish hen in half and serve with mashed potatoes, rice, or roasted/steamed vegetables. Drizzle some sauce, and enjoy!