Melt the butter in a medium saucepan over medium heat. Add the minced garlic and cook for about 30 seconds until fragrant.
Sprinkle the flour over the butter and whisk continuously for about 1 minute to create a smooth base for the cheese sauce.
Slowly pour in the milk while whisking constantly to prevent lumps. Continue cooking for 3 to 5 minutes until the mixture begins to thicken.
Reduce the heat to low and gradually add the shredded cheddar and Monterey Jack cheese. Stir continuously until the cheese melts completely and the sauce becomes smooth and creamy.
Add the diced tomatoes with green chilies, chopped green chilies, cumin, chili powder, salt, and black pepper. Stir well until all ingredients are evenly combined. (dinnerthendessert.com)
Continue cooking for another 1 to 2 minutes until the queso reaches your preferred consistency. If the dip is too thick, add a splash of milk and stir until smooth.
Remove from heat, stir in fresh cilantro, and transfer to a serving bowl. Serve warm with tortilla chips or your favorite dippers.