Preheat oven to 400 °F. Spray a 9 1/2-inch pie plate with cooking spray. Heat the half and half and chicken stock in a small saucepan over medium heat. Whisk in the fresh herbs, garlic, dry mustard, and nutmeg and bring to a simmer. Remove from heat. Trim the turnips of any root ends, peel, cut into 1/8-inch slices and set aside. Peel the onion and cut into 1/4-inch slices, leaving rings intact. Layer bottom of the baking dish with half of the turnip slices, overlapping slightly. Lightly salt and pepper the first layer. Evenly layer all of the onion slices on top of the first layer of turnips. Lightly salt and pepper the onions. Layer the remaining turnip slices on top of the onions, overlapping slightly. Lightly salt and pepper them. Pour the cream and herb mixture over the layered vegetables in the pie plate. Stir together the parmesan and panko in a small bowl and sprinkle evenly over top layer of turnips in baking dish. Cook for one to one and a half hours, until a knife easily penetrates vegetables, the top is browned, and most of the liquid is absorbed. (If your topping is browning too quickly, you can cover baking dish lightly with a greased sheet of foil.) Let gratin cool at least 10 minutes before slicing.
