Cook chorizo, drain and set aside.
Melt butter in saucepan. Whisk in the flour, and allow to cook for a couple minutes.
Whisk milk into butter/flour mixture. Cook for about 2-3 minutes, until mixture thickens.
Reduce heat to low. Add your cheese (shredded or cubed) and tomatoes, and stir until cheese is melted.
Mix in cooked chorizo.