IdahoSandy

I always pre-cook mine in the microwave for 10/15 minutes with very little water(have to watch water level), then add the mushroom soup and etc.

JBeamer

Peel potatoes; slice thin. Arrange in baking pan 9x12 or what ever size you have on hand. Put 1/3 of onion peices on top of potatoes. Dot with real butter. Sprinkle with salt and pepper. Sprinkle with flour. Repeat with two more layers. Be careful not to add too much onion or pepper. When pan is 3/4 full of potatoes, add milk to cover. Cover with foil and cook about 1 to 1 1/2 hour or till fork tender. This makes it's own white sauce. You can add grated cheese for au gratin or top with pork chops. If using pork chops uncover and turn for the last 30 minutes.Temp. about 350 or so.. I don't measure but I think 3 large potatoes, one half thin sliced onion separated into rings and cut in to roughly quarters. The amount of flour is maybe 1 tablespoon per layer. Makes 4 large servings.

marmalade

You may want to consider lowering your oven temperature to 325*, if the recipe calls for 350*.
Usually when you bake something in a glass pan, they suggest to lower the oven temperature (I think because the glass retains the heat more?)