Grate the cheddar and Monterey Jack cheeses. It's crucial to use freshly shredded cheese as pre-shredded varieties contain anti-caking agents that can make your queso grainy. Dice the fire-roasted Hatch chiles and set aside. Warm the whole milk gently in a separate pan or microwave.

In a large saucepan or Dutch oven, melt the unsalted butter over medium heat. Once melted, sprinkle in the all-purpose flour and whisk constantly for 1-2 minutes until a light golden roux forms. This will be the thickening agent for your queso.

Gradually whisk in the warmed whole milk, a little at a time, ensuring there are no lumps. Continue whisking and cooking for about 3-5 minutes, until the mixture thickens to a consistency similar to a thin gravy.

Reduce the heat to low. Stir in the ground cumin, garlic powder, onion powder, cayenne pepper (if using), salt, and black pepper. Mix well to combine the spices.

Add the cubed cream cheese and stir until completely melted and smooth.

Begin adding the shredded cheddar and Monterey Jack cheeses in small handfuls, stirring continuously until each addition is fully melted before adding more. This slow, gradual process helps create a smooth, lump-free queso. Do not allow the mixture to boil once the cheese is added, as this can cause it to separate.

Once all the cheese is melted and the queso is smooth and creamy, stir in the diced fire-roasted Hatch chiles. Cook for another 1-2 minutes to allow the flavors to meld.

Transfer the warm queso to a serving bowl. Garnish with chopped fresh cilantro and finely diced red onion, if desired. Serve immediately with plenty of tortilla chips for dipping.