Chop the potatoes into cubes and set aside.
Cut the bacon into strips and sauté in a pan until crisp. Remove from pan to drain on paper towels.
Add the butter to any bacon fat left behind.
Add the chopped onion and garlic to the pan and sauté until golden.
Add the flour and form a roux paste, stirring constantly.
Slowly, pour the milk in a little at a time while you continue stirring, to avoid lumps.
Heat the sauce over medium high heat until smooth.
Add 1 cup mozzarella or Monterey Jack cheese, and stir until the cheese has melted.
Add most of the chives, reserving a tablespoon full for garnishing the soup when it is served.
Add the potato cubes and most of the bacon, then gently mix as the soup heats through.
Stir in the sour cream.
Serve topped with a sprinkle of grated cheese, chives. and the reserved strips of bacon.