Add the evaporated milk to a saucepan and warm up over medium-high heat. Immediately add the cornstarch and whisk until combined.

Once it begins simmering and bubbling (but before boiling), turn the heat to medium-low.

Add the American cheese, about 2 slices at a time, stirring constantly. Once each batch melts, add another few slices. Be sure to never stop stirring.

Once all the cheese has melted, add the Hatch chiles, salt and onion powder. Stir to combine.

Remove from heat and serve immediately.