Make roux. Melt the butter over medium-low heat in a large saucepan. Whisk in the flour and continue to cook, whisking constantly, until flour smells toasted but not browned.

Add garlic. Add the garlic and cook another minute, whisking frequently.

Slowly add milk. While whisking constantly, gradually add milk, a little at a time, letting the first bit get soaked up completely by the flour before adding the next. Keep adding until all milk is mixed in. Adding the milk too quickly will result in a clumpy dip so go slow. Bring to a simmer, then remove from heat.

Add cheese. Stir in cheese, chiles, and salt then taste and add more salt if needed.

Serve. Transfer to a bowl and top with scallions, remaining chiles, and cilantro. Serve immediatley with tortilla chips.