Melt butter in medium pot at med/low heat, then add onions and peppers, frying until onions are translucent
Add garlic, cook for one minute
Whisk in flour a bit at a time, slowly cooking out the flour, about 1 ½ minutes
Add milk bit by bit, whisking constantly until smooth & combined, about 5 minutes
Turn heat to low, add cheese ¼ cup at a time, stirring the while time
Make sure ALL the cheese is combined with NO lumps before adding the next ¼ cup
Repeat with all the cheese
Stir in sour cream & cumin, salt & pepper to taste