In a large skillet, cook the ground sausage over medium heat until browned. Add the chopped onion and celery and cook until softened, about 5 minutes. Stir in the minced garlic and cook for another minute until fragrant. Remove from heat and transfer to a large bowl. Add the bread cubes, fresh herbs, and chicken broth, tossing gently to combine. Season with salt and pepper to taste. Set aside to cool slightly.

Preheat your oven to 400°F (200°C). Pat the Cornish hens dry inside and out with paper towels. Season the cavity and the exterior of each hen generously with salt and pepper. In a small bowl, mix the softened butter, minced garlic, and chopped parsley to create the garlic herb butter.

Gently loosen the skin over the breast meat and spread half of the garlic herb butter under the skin of each hen. Fill the cavity of each hen with the prepared sausage stuffing. Secure the legs together with kitchen twine.

Place the stuffed hens in a roasting pan. Rub the remaining olive oil over the skin. Roast for 25 minutes, then reduce the oven temperature to 350°F (175°C). Continue roasting for another 30-35 minutes, or until a meat thermometer inserted into the thickest part of the thigh reads 165°F (74°C).

Remove the hens from the oven and let them rest for 10 minutes before carving. Serve whole.