Add the evaporated milk to a large saucepan and place over LOW heat. Whisk in the cornstarch until completely dissolved. Add the onion powder and salt and whisk to combine. Add the cheese and increase the heat to MEDIUM. Cook, stirring frequently with a wooden spoon, until the mixture is hot but not boiling, then reduce the heat to LOW and continue to cook, stirring frequently, until the cheese has melted and the mixture is smooth and thickened. Remove from the heat.
For more flavor and a mild kick, add hot sauce and stir it into the warm cheese sauce, off the heat. I add about 1 tablespoon or more. It doesn’t add much heat, just a kick and some great flavor. Taste and add additional salt, only if needed. Since it’s typically served with salty tortilla chips, I find this is enough salt.
Once cooled, transfer to an airtight container and refrigerate for up to 1 week. This stuff reheats great! Can be made well in advance and reheated, as needed for nachos or dip.
For nachos: Pour over nachos while hot and the consistency is sauce-like.
For a dip: As the sauce begins to cool, it will thicken more and work well for a dip. Just whisk it occasionally if a skin forms on the surface.
To reheat: Place in the microwave, covered with a paper towel, and cook in 20 second increments, stirring in between, just until warmed through. Or, reheat it in a small saucepan over LOW heat on the stove. The sauce will loosen up when you reheat it.