Evaporated milk is milk that has had most of the water removed. It makes the dish more creamy without adding a lot of calories and fat. If regular milk is all you have on hand, pour 1/2 a cup of milk into a small saucepan, then add an additional 2/3 cup of milk. Simmer the milk over medium low heat until the milk has cooked down to the required 1/2 cup. If you don't want to go through all this, you can just dump 1/2 cup of milk in there and move on with your day. But be advised, your casserole may not come out as good as mine. :)