Make a roux using butter (full fat) and flour. Slowly add the milk (full fat) until the sauce has thickened. Grate a huge amount of cheese (full fat) and stir in. I like plain old cheddar- this isn't the place to be getting fancy. Add some almost cooked pasta (preferably yummy yellow french stuff) and bake, or grill until crispy on the top. I love it with tenderstem broccoli.