In a saucepan, combine the salt, brown sugar, peppercorns, sage, garlic, thyme, rosemary and 1 quart water. Bring to a boil. Cook and stir until salt and sugar are dissolved. Remove from the heat. Add the cold water to cool the brine to room temperature., Place a turkey-sized oven roasting bag inside a second roasting bag; add hens. Carefully pour cooled brine into bag. Squeeze out as much air as possible; seal bags and turn to coat. Place in a roasting pan. Refrigerate for 1-2 hours, turning occasionally. Drain and discard brine; pat hens dry., Meanwhile, place the peppercorns and garlic in a food processor; cover and pulse until coarsely chopped. Add the butter, 3 tablespoons olive oil and herbs; cover and process until smooth. With fingers, carefully loosen skin from hens; rub half the butter mixture under skin. Secure skin to underside of breast with toothpicks; tie drumsticks together. Rub remaining butter mixture over skin., In a 15x10x1-in. baking pan, toss the Brussels sprouts and onions with remaining olive oil, salt and pepper. Arrange in a single layer. Place hens, breast side up, on top of the vegetables. Bake at 450° until thermometer inserted in breast reads 165°, 35-40 minutes. Cover loosely with foil if hens brown too quickly., Remove hens and vegetables to a serving platter; cover and let stand for 10 minutes before carving. If desired, garnish with additional herbs.