In a medium sauce pan over medium heat, melt butter. Add flour, and whisk until smooth. Slowly add milk. Whisk until smooth and bubbly. Stir in cheese, and stir until melted and smooth.
Add Rotel. Mix to combine. Add salt and pepper to taste.
Notes
NOTE: Rotel is just diced tomatoes and green chiles. If you can't find it, you can do about 7 ounces of diced tomatoes (half of a can) and 3 ounces of green chiles (half of a small can).