Unlike say nesselrode, haupia, or ropa vieja, where the name of the food gives absolutely no clues as to how it is made or what it is, "evaporated milk" gives some really good clues. It's milk that has been evaporated. And if you can't run to the store during an emergency shortage, given a little time, you can easily make some yourself or try one of several other options.

Store-bought evaporated milk has had about 60% of its water removed through evaporation. Because it is a concentrated form of milk, it is often called for in recipes to add richness to a dish. And because it is relatively thick, it is called upon to contribute to a creamy texture. In the case of your chile rellenos, both of these qualities may have been sought. In addition to flavor, the recipe writer may have specified evaporated milk to help the eggs glom the flour to the peppers (we believe glom is the correct cooking term in this case).

In the absence of evaporated milk, one option is to take some dry milk and reconstitute it using only 40% of the recommended water. That would give you the right texture. It is difficult to find dry whole milk (because the fat in it tends to spoil and so should be refrigerated), so you would most likely be using dry nonfat milk, which would cause some adjustment in flavor.

Or you could take a page from Southern cooks and use buttermilk, which is often used in the breading process for fried chicken. It has the thick texture to help the coating adhere and certainly gives a boost to the flavor, although it might be more on the tart side, where evaporated milk leans towards sweetness.

You can also use some cream instead of evaporated milk, although you might find that too rich.

If you have a little time, though, you can easily reduce a quantity of milk by half or more. Put it in a pan on the stove, bring it to a simmer, and let it reduce. A wide-mouth pan such as a saucier will allow more surface area for faster evaporation. Keep the milk below the boil, to keep it from curdling, and make sure it doesn't scorch on the bottom of the pan, or you'll have a tough time scrubbing it off.