Grease casserole dish with butter. Layer the casserole dish with one layer of peeled, sliced potatoes and one layer of sliced onion, another layer of potatoes and another layer of onion. (4 total layers). Add several dabs of butter to top layers, sprinkle with a little salt. Add milk to casserole until milk covers ½ of the layers. Bake at 350°F for 1 hr. Browning is inevitable, and good.
If you want more sauce, add a bit of milk while cooking (you can tell how much will absorb).