Begin by grating the mozzarella and American cheese separately.
In a medium pan over medium low heat, add the chopped shallot and stir. Add in milk, then take a small handful of the grated American cheese and use a whisk to stir it in till smooth. Be patient, the lower heat keeps this nice and smooth, but if it's not melting at all turn up the heat slightly. Add American cheese a small handful at a time till completely whisked in.
Continue the process with the mozzarella cheese, adding in one small handful at a time and whisking in between. Once the mozzarella is whisked in completely and smooth. Add in any mix ins you like - cumin, the green onion, cilantro, and any salt and pepper to taste (I usually don't add any). Serve immediately.
Notes
For American cheese I recommend Andrew and Everett, Boars Head, and Organic Valley. You can ask the deli counter to cut it into a block for you, or smush a bunch of slices together and grate it that way.
I recommend using whole milk for the best texture and flavor, but any milk is ok. Just watch the heat or lower fat milks can curdle more easily.
To reheat, just place back on low heat, or microwave in 15-20 second intervals, stirring between.
Serve with tortilla chips, tortillas, tacos, taquitos, or my Mexican stuffed sweet potatoes!