Preheat the oven to 375 degrees F. With kitchen shears or a knife, cut along either side of each hen's backbone and remove. Press the hens flat, skin-side down. Cut each in half down the middle of the breast bone. Place the hen halves skin-side up in a roasting pan. Combine the garlic, butter and sage in a small bowl, brush on the hens. Season with salt and pepper. Roast 45-50 minutes, or until cooked through.
Meanwhile, heat 2 Tbsp. of the oil for the risotto in a pot set over medium heat. Add onion and cook until tender. Add the rice and cook, stirring, until it achieves a nutty, toasted aroma, about 2-3 minutes. Add the wine. Adjust temperature so that the liquid gently simmers. Cook until the wine is almost fully absorbed by the rice. Add 1 cup of stock, stirring and cooking until is almost fully absorbed. Add remaining stock 1 cup at a time, cooking and stirring until it is almost fully absorbed each time. As you do this, heat the remaining oil in a large skillet over medium heat. Add the mushrooms and cook, stirring, until tender, about 5 to 6 minutes, then remove from the heat. When the rice is tender, mix in the mushrooms, cheese, sage, salt and pepper and heat through. Divide and spoon the risotto in 6 bowls or plates. Top each with a half Cornish hen. Garnish, if desired, with sprigs of fresh sage and serve.