- In a medium saucepan, melt the butter over medium heat.
Add the diced onion and cook until soft and translucent, about 2–3 minutes.
- Stir in the garlic, green chiles, cumin, salt, and pepper, and cook for another minute.
- Sprinkle in the cornstarch and stir until fully incorporated.
- Slowly whisk in the milk and bring to a gentle simmer, stirring constantly.
- Reduce the heat to low and add the white American cheese and Monterey Jack cheese gradually, stirring until fully melted and smooth.
- Stir in heavy cream if desired for extra richness.
- Keep warm and stir occasionally until ready to serve.
- Garnish with chopped cilantro or jalapeños if using, and serve with tortilla chips.