In a medium saucepan, warm the oil over low heat. Add the onion, tomatoes, garlic, chili powder, paprika, salt, and cayenne and cook, stirring occasionally, until the tomatoes have almost completely disintegrated, 6 to 8 minutes.
Whisk together the flour and the water to make a paste, then add it to the pan. Stir a few times until the paste is well combined with the vegetables. Stirring constantly, add the cheese, which should combine quickly. As the cheese melts, the queso will become thick and almost like putty. Don’t be alarmed! This is the proper texture as it is not a creamy queso. Taste and adjust the seasonings, if you like.
Transfer the queso to a serving bowl, a small slow cooker, or a chafing dish over a flame. Serve immediately with tortilla chips.
Notes
One of the characteristics of this queso is that the oil separates from the cheese. The amounts of oil used in the various recipes were all over the place but I found ¼ cup provided enough without it being overly greasy. Though if you like it super greasy feel free to add 2 tablespoons more oil.