Preheat the oven to 375 degrees F. With kitchen shears or a knife, cut along either side of one of the hen's backbone and remove. Press the hen flat, skin-side down. Cut in half down the middle of the breastbone. Place the hen halves skin-side up in a roasting pan. Repeat with the other hens; season with salt and pepper.
Roast the hens 55 minutes, or until cooked through. When the hens are almost done, place the citrus juice and zest, stock, brown sugar, vinegar, soy sauce, cornstarch, ginger and garlic in a pot and whisk to combine. Simmer a few minutes and then cover and reserve on low heat until the hens are ready.
When the hens are cooked, serve them with the sauce; garnish with the chopped mint or parsley and citrus slices.