To prevent a soggy bottom on your Beef Wellington, make sure your duxelle is cooked well to let the mushrooms release all their moisture. Also, the prosciutto will help to act as a barrier to keep the liquids inside. Brushing an egg wash on the inside of the pastry can also help form a barrier while it cooks. And last, don’t forget to cut diagonal slashes every inch or so down the pastry to allow steam to vent while the Wellington is baking. This will prevent a build up of moisture.