From what you're describing, I think the cheese (or cheese-like substance, in this case) 'broke' ... that is, the oil separated out of it.
I'd recommend:
- Don't melt it at full power
- Stir it multiple times during melting
- Melt the cheese (or "cheese-food") seperately, then add the tomatoes, so the tomatoes don't get hot, causing uneven heating.
I'm not sure if there's a good way to recover cheese once it's broken. Some starch will help keep dairy from breaking in the first place, but I don't think it'd help you in this particular case.
If you want more flavour, add more flavour. That sounds like I'm being a smartass, but it's true. Starches absorb/obscure flavour, which is a large part of why both the nouvelle cuisine and the molecular (or whatever you want to call it; I hate that term) movements prize reductions and non-standard thickeners, respectively.
First of all, I'd echo Michael and suggest you not use a roux. That will instantly create more flavour (this is assuming you're keeping the roux blonde, and not cooking it darker for more intense flavour).
Second, you will want to introduce a liquid phase to the dip to keep it dippable as it cools while you eat it. I would suggest cream, or sour cream. Buttermilk would also be lovely for a nice tang. You won't need much, I would just substitute for the milk you are already using, and only use one cup to begin. Add more as needed to adjust consistency. Bring your liquid product to a slow simmer, and slowly whisk in your cheese until incorporated.
For the cumin, garlic, chiles, I would start by toasting the cumin in a hot dry pan, then add your olive oil, sautee the onions first (or even go so far as to caramelize, which would be lovely), then add and lightly cook the chiles and garlic. This will bring out more flavours.
I don't know what you mean by 'Queso' cheese; Queso is just Spanish for 'cheese.' Queso fresca? Queso blanco? But I would look for cheeses with more flavour. The ones you are using are fairly bland (comparatively) and will not provide an intensely cheesy experience.
Season at the end.
I would first, however, simply try making the same thing without a roux, and using half the milk. The flavour difference will be significant.
Best Answer
If we're going to define "cheese dip" as "some substance made from cheese for the purpose of dipping other food in" - which seems like the only rational definition given no other context - then I'd have to pinpoint its origin as being fondue.
According to Wikipedia, the earliest known recipe for fondue comes from the Swiss cookbook, Kochbuch von Anna Margaretha Gessnerin, ein gebohrne Kittin, first published in 1699.
That predates the Arkansas claim of 1935 by at least a few centuries, if we assume that Wikipedia does indeed list the earliest source.
There's also Queso Chihuahua which was supposedly brought to Mexico by the Mennonites, which would place its origin in Mexico sometime in the 1920s - assuming that the Mennonites themselves did not simply adapt the recipe from something known even earlier.
I'd have to say that the Arkansas tourism board seems to be trying to pull a fast one on us, laying claim to this "invention". Maybe they're defining cheese dip as the specific kind of cheese dip they came up with, which would of course be a tautology.