Preheat oven to 350 degrees.

Make the white sauce. Melt the butter (I melt it in the microwave, but you can do this on the stove top as well), and mix in the flour. Then whisk your butter and flour mixture into a medium sized bowl with the milk. Last add Salt and Pepper. White sauce DONE!

Peal and thinly slice your potatoes and onions.

In a greased casserole dish, layer half of your sliced potatoes, then half of the sliced onions, and then pour half of the white sauce on top.

Repeat these layers one more time... second half of sliced potatoes... second half of sliced onions... and pour on the last of the white sauce.

Cover with tinfoil and bake in the oven for 1 hour. After an hour remove the tinfoil, and bake for another 30 mins uncovered. DONE!