First, make a simple roux: Melt the butter in a medium sized saucepan over medium high heat. Sprinkle in the flour, and quickly whisk for 2 minutes until golden. Slowly whisk in the milk a little at a time, and stir until combined and there are no lumps.
When the mixture is thick enough to coat a spoon, stir in the cheese, salt, pepper, dry mustard, and cayenne until smooth. Once all the cheese is melted, remove the saucepan from heat and add the diced tomatoes and jalapenos, if using. Transfer the contents to a heat-safe bowl and serve hot with tortilla chips.
Notes
The Jalapeno Option: I recommend using diced candied jalapenos (otherwise known as Cowboy Candy) in this recipe, as it adds a nice spicy sweetness. If you don’t make your own preserves or can’t find any though, canned diced jalapenos would work just fine. If you decide to use fresh jalapenos, I recommend dicing them up and sauteing them in a bit of oil to soften them up before adding them to the cheese mixture. Of course, you leave them out all together for a mild queso dip.