In a medium saucepan, melt the butter over medium heat. Add the finely chopped onion and sauté for about 2-3 minutes until the onion turns translucent. Then, add the crushed garlic and sauté for another minute until fragrant.
Sprinkle the flour over the sautéed onions and garlic. Stir continuously for about 1 minute to form a roux, which will help thicken your dip.
Gradually pour in the whole milk while whisking vigorously to combine. Continue to stir until the mixture thickens, which should take about 3-5 minutes.
Reduce the heat to low and add the shredded Monterey Jack cheese to the saucepan. Stir until the cheese is fully melted and the mixture is smooth.
Once the cheese is melted, fold in the diced Roma tomato, canned green chilis, and chopped cilantro. Mix until everything is well combined.
Transfer the Queso Blanco Dip to a serving bowl and enjoy it warm with tortilla chips, veggies, or any of your favorite dippables!