Traditional:

1. In a medium saucepan, heat the oil over medium heat until almost smoking.
2. Add the onion and jalapenos, and season with salt. Cook until softened, about 2-3 minutes, stirring occasionally.
3. Add the garlic and saute until softened, about 2-3 minutes. Stir in the tomato and cook until just hot. 
4. Add the American and pepper jack cheeses, and the milk. While stirring constantly with a wooden spoon, cook over medium heat until silky smooth. 
5. Season with salt and stir in the cilantro. 
6. Transfer to a serving bowl and garnish with tomato and cilantro. 

Fancy:

1. In a medium saucepot, melt the butter over medium heat. 
2. Add the shallots, chilies and garlic, and season with salt and pepper. Saute until softened, about 2-3 minutes, stirring often.  
3. Sprinkle in the flour and cook for 30 seconds, whisking frequently. 
4.  Whisk in the milk and continue to stir constantly until the mixture thickens.
5. Combine the cheddar and gruyere cheeses in a medium bowl. While whisking, add the cheese blend in small batches. Turn off the heat and continue to whisk until the cheese has completely melted and the mixture is smooth.
6. Whisk in the sour cream, and taste to determine if more salt and pepper is needed.
7. Place a generous amount of avocado in a serving bowl and season it with flaky salt. Pour the hot queso on top, covering the avocado completely. Garnish with the pickled red onion, thinly sliced chilies, and cilantro leaves. Drizzle with hot sauce.