CHEESE PREP: Grate cheese or coarsely chop if already sliced. If you don't have an exact weight, measure to 2-1/2 cups. Toss cheese pieces with cornstarch in a small bowl and set aside.
GREEN ONIONS: Heat stovetop to medium heat. Melt butter in a medium-sized pot. Once melted, add in the thinly sliced green onions and cook, stirring occasionally for 3-5 minutes or until soft. Add in the garlic and stir for 30 seconds.
SPICES AND CHILES: Thoroughly drain the can of green chiles. Add all the green chiles, spices, and salt & pepper (add to your taste preference; I add 1/2 teaspoon of each) to the pot. Stirring, cook for 1-2 minutes or until very fragrant.
MILK: Give the can of evaporated milk a good shake and pour into the pot. Stir and bring to a simmer still over medium heat. Do not increase the heat and don't boil! Once simmering, reduce the heat to medium low.
CHEESE: Add in the cheese and cornstarch mixture and stir constantly until cheese is melted. This takes a bit of time. You may be tempted to increase the heat, but don't -- this is how you get a grainy dip. Once the cheese is fully melted (sauce will still seem a bit thin, but it thickens as it sits!), taste with a tortilla chip so you can gauge saltiness. Adjust any seasonings/salt to personal preference. Stir in cilantro if using. Remove from heat and serve warm with chips.
KEEP IT WARM: Dip will thicken and harden as it cools (just like any melted cheese-filled recipe). It's great right off the stovetop, but will get thicker and thicker the longer it stands. Here's how to keep it nice and creamy for a longer serving period: Transfer to a small crockpot (ideally 2-4 quarts) and set it to "warm". Serve it right out of the slow cooker, stirring it every now and again. Alternatively, when it gets too thick, re-warm over low heat in a pot on the stovetop or microwave (covered) in bursts of 10 seconds until warmed through. If needed, add an extra splash of evaporated milk (while stirring constantly) as you re-heat the dip.
STORAGE: Queso will store nicely in the fridge for 3-5 days. Keep it in an airtight container and use the re-heating methods described above. Queso doesn't freeze/thaw well.