In a large pot, melt 1/2 stick of butter, and add garlic and sliced onions.
Set temp to medium high so that the onions will cook and brown quickly.
STIR OFTEN!
Onions should be cooked until they are a very dark brown, but not burnt. This causes them to "caramelize" and release their sweet flavor.
Allow the onions to cool down to warm before adding beef broth.
Return soup to heat and allow to simmer.
Add pepper to taste.
You can add salt, but this usually isn't necessary since the broth will be salty enough.
Butter several slices of your favorite sourdough bread and sprinkle with one or any combination of the suggested cheeses.
Place under the broiler until the bread begins to brown and the cheese is bubbling.
Ladle soup into bowl and top with slices of bread.