Make a roux with the butter, flour and milk.
Add the garlic, onion, and chili powder. Add cheese and stir until is melted and smooth.
Stir in cilantro and tomato immediately before serving.
Made this several weeks ago. It definitely needed added salt, but I don't recall how much we ended up adding. I believe it was also thinner than we preferred so we added additional cheese as well, but that could have been addressed initially by making a roux with the proper proportions of butter to flour (1:1).
Made this without cilantro; white American cheese; half heavy cream and half water. Delicious, but perhaps slightly too salty. I did not measure the cheese, but added as much as I pleased.