Soggy French Toast usually just needs a little more baking time. However, a lot depends on the type of bread you're using and the ratio of custard ingredients to bread. Use a dense, sturdy bread with body, like a brioche or a chewy French bread. Limp white bread will not make good French toast. If you find there is a lot of liquid in the bottom of the baking dish when you're ready to bake the casserole, drain most of it off. The bread should have soaked up most of the liquid as it sat in the fridge.