Start by melting up 3 tbsp of butter. Grate up that cheese while the butter melts;

Once the butter is liquefied it's time to form the roux so dump in the flour and mix it up so that it looks like this - this is a white roux so use low heat and avoid cooking it brown.

Once it looks like this dump in the milk:

This is where you have to stand vigilantly over the process - don't let it cook on its own, keeis bad and so is the formation of milk scum. If your sauce looks like it does in the pictures here, you are good;

Getting there;

OK now the butter droplets go away and it starts to get smooth and thicker so DON'T STOP STIRING, this is where you can mess up by burning the milk at the bottom and getting nasty sauce.

KEEP STIRING!

It's good to see some bubbles.

There we go - it sticks to your wooden spoon, it's smooth, white and thick.

Now grab your cheese and dump it on in there (I'm using yellow cheese because that's all I had that day - swiss is better).

Cheese starting to melt.

Cheese melted: After the sauce is looking good, salt and pepper to taste.

Have your sliced potatoes close by. What I do is take uncooked potatoes and slice em about 1/2 inch thick. Grab something fireproof to cook in and line the bottom with potatoes, then a layer of sliced onions (again uncooked), and repeat until the cooking device is full as seen above! Of course this should be done before you make the sauce but if you forget, no problem, the sauce can be reheated if need be.

Dump the sauce over the potatoes.

You can also dump on 1/2 cup of bread crumbs you've cooked up with some butter, salt and pepper at this point if you like a crispy top. Since we are using raw ingredients we want to cook this for around an hour at 400. I usually cover it with aluminum foil and check it at an hour - sticking a fork in it should be almost effortless if the potatoes are cooked up right. If they aren't soft just leave it in there some more.

Now that you have learned how to make cheese potatoes, please be sure to view these other cheese recipes.