Toss potatoes, shallots, oil, thyme, rosemary, pepper, salt, 3/4 cup Gruyère and 1/3 cup Parmesan together in a large bowl. Arrange in a decorative layer in the prepared dish. Pour broth evenly over the potato mixture. Sprinkle the top evenly with the remaining Gruyère and Parmesan. Cover tightly with foil.