Gratin help!

Or if it's too soupy after more cooking, spoon the more solid parts into another baking dish, top with bread crumbs, finish baking.

Do you have time to leave it in the oven a while longer to let more of the moisture evaporate? It will set up a bit more as it cools anyway, but if it's quite sloshy, I think I would maybe try to spoon any excess liquid away and keep baking it. If the top starts to brown too much, you could move it lower in the oven, or loosely cover with foil. How's that plan sound?