Cheese sauce is usually made with flour to create a roux, but since flour is chametz I substituted it with potato starch instead so you can enjoy a good cheese sauce even on Pesach!
In a small saucepan over medium heat, melt butter. Once completely melted, add the potato starch while mixing with a spoon. As soon as that is combined, add the milk and stir again to break any lumps.
Allow milk to come to a boil, stirring every minute. Once it comes to a boil, add the kosher salt and cheese and allow it to melt. Stir occasionally to help the melting process.
Once the cheese is melted, remove from heat and serve. Store in an airtight container or a loaf pan in fridge.
The cheese sauce can be prepared and rewarmed in the oven. Cheese sauces are good cold, but they are even better hot!