Put salsa in a small strainer to get rid of the watery juice.  

In a small saucepan, melt butter over medium-high heat. Add the flour and whisk to incorporate. Let the butter and flour mixture cook, whisking regularly until it has browned slightly. Turn heat down to medium-low.  Add a small amount of milk and whisk to combine. It will thicken rapidly. Continue adding the milk and whisking to combine until all the milk is incorporated.

Turn down the heat to low.  Add the grated cheese one handful at a time, stirring until it’s completely melted. Continue adding cheese in small handfuls until it’s all used up.  If the heat is too high, the cheese can take on a grainy appearance.  It still tastes fine, it just doesn’t look as good!   Remove from heat and add salsa and salt and stir until just combined.