In large Dutch oven, sauté onions in butter or margarine. When they are almost transparent, add garlic. You don't want to over cook the garlic.
When tender, add broth and potatoes and bring to boil. Turn down to simmer. Stir often, until potatoes are tender
Add as much milk for the amount of soup you want to make. Usually about 2-3 cups.
When new mixture comes to a simmer, add cheese-cubed, and cream cheese-cubed, stir often to keep from sticking, when boiling this time, reduce heat and simmer until it is as thick as you want it.
To thicken, you can add cornstarch mixture (1/4 cup cold milk and 2 tbsp corn starch, blended well).
