Roast hatch peppers over an open flame (a gas stove top will do) for about 2-3 minutes per side, until skin is charred.
Scrape skin off hatch peppers, remove seeds, and chop into 1/8 inch pieces. Set aside
In a skillet, melt butter, garlic, and grated onion together. Stir in flour until thick. cook for 2 minutes.
Gradually whisk in milk and bring to a simmer. stir in cream cheese, Monterrey, and cheddar cheese, until melted.
Stir in hatch pepper and reduce heat to low. Let it cook for about 5 minutes, constantly stirring. At this point add more milk, to get your Queso to the consistency of your liking.
Top with green onion, and serve along with chips. Enjoy!