1. Preheat the oven to 375°F and lightly grease a baking dish or roasting pan.

2. Melt the butter in a skillet over medium heat, then cook the onion and celery until softened, about 5 to 6 minutes.

3. Add the garlic and cook for 30 seconds, just until fragrant.

4. In a large bowl, combine the bread cubes, cooked vegetables, thyme, rosemary, parsley, salt, and black pepper.

5. Pour in the chicken broth gradually and mix gently until the stuffing is moist but not soggy.

6. Pat the Cornish game hens dry with paper towels and rub them all over with olive oil.

7. Loosely fill the cavities of the hens with the stuffing mixture.

8. Place the stuffed hens in the prepared baking dish and tuck the lemon halves alongside them.

9. Roast for 55 to 65 minutes, or until the skin is golden brown and the thickest part of the thigh reaches 165°F.

10. Let the hens rest for 10 minutes before serving.