Pour 4 quarts of water into a large pot, cover and set over high heat.
In a medium-size saucepan, melt the butter over medium heat, then stir in the flour and whisk until the mixture turns a deep golden, about 2 minutes. Add the milk and continue to whisk constantly until the mixture thickens and comes to a full boil, 4 or 5 minutes. (Whisk diligently and there will be no lumps.) Stir in the salsa, remove from the heat, then stir in cheese. Continue stirring until all the cheese has melted. Taste and season generously with salt, usually 1 1/2 teaspoons. Cover to keep warm until the pasta is done.
When the water has come to a boil, add the dried pasta. Stir well to keep the pasta from sticking together and boil until the pasta is al dente, about 10 minutes. Drain thoroughly and add the pasta to the sauce. Stir until all the pasta is covered in the salsa cheese sauce. You’re ready to dig in.