Make white sauce of first 5 ingredients*
Place half the potatoes in greased 2-quart casserole; cover with half the onion and half the sauce. Repeat layers. Cover and bake at 350 about 1 hour. Uncover and bake 30 minutes longer.
* Shredded cheese may be added to sauce.
I used to use two glass oblong dishes, and more potatoes to accommodate more people. So I would improvise on the rest of the ingredients too. You can use onion powder also. When you make the sauce, put the flour & cold milk in something that you can shake it up in. If you don't have that, you can just add flour to the milk very gradually while stirring with a fork. (The idea is to blend these cold first so that you don't get lumps when sauce starts to heat). Then pour in a pan, add other 3 ingredients (& cheese if using). Put on low heat, stirring often to prevent lumping.
The recipe didn't really say how to make the white sauce. Also, I don't think I ever cooked uncovered at any point? You don't want to fill dish too full, or it will overflow in oven. (You could put foil underneath, just in case). Also, sometimes I ended up having to cook longer than it called for. Oh....and I used baking or russet potatoes.