In a large stockpot over medium heat, first heat the butter and oil, then add the sliced onions and salt; saute the onions until they are browned. This will take up to 30 minutes; you want the onions to have the color of caramels. Stir frequently to prevent burning.
When the Onions are almost fully caramelized, add the chopped garlic.
Cook the onion and garlic together until the garlic has become fragrant, about 2 minutes. Add beef broth, beef base, and black pepper. Taste and add more salt if needed. Simmer for 30 to 45 minutes over low heat.
To serve soup, preheat the oven to broil. Ladle soup into 8 individual ovenproof bowls, place a slice of bread on top of each, add top each slice with 1 teaspoon Parmesan cheese, and provolone cheese. Place under the broiler and cook until the cheese just begins to brown.