1. I use a 2 1/2 quart covered casserole dish for this. This will stick unless you have a great non-stick baking dish - I never grease the pan though because of all the fat that's already in this.
2. Preheat your oven to 350.
3. Brown hamburger and onion.
4. Place a layer of sliced potatoes in the casserole dish, top with half of the hamburger/onion mixture. Salt and pepper to taste (kind of depends on how salty your cheese sauce is - you can always salt it later if it needs it).
5. Repeat layering of potato slices and hamburger mixture (and salt and pepper if desired).

Make Cheese sauce:

1. Melt 3 T butter in saucepan over med-high heat. Add 3 T flour, and the salt and pepper. Stir quickly to blend. Add 2 cups milk; stir with whisk to incorporate butter/flour mixture. Reduce heat, stir till thickened and bubbly.
2. Remove from the heat and add the shredded cheddar. Stir until the cheese melts.
3. Pour the cheese sauce over the hamburger and potatoes in the baking dish. Push the potatoes around to let cheese sauce into all of the crevices so it's well coated.
4. Cover and place in the oven. Bake for about 1 1/2 hours, till the potatoes are tender and the top is browned. I like to take off the cover for the last 20 minutes or so to brown the top a bit.
5. Even with the leanest ground beef, this can get little pools of grease on top - I just spoon these off. It doesn't mean the whole thing is greasy - just cheesy.