Cook 2 tablespoons butter and olive oil in 6-quart sauce pot over medium heat until butter is melted.  Add onions and salt and cook 20 minutes or until onions are tender, stirring occasionally.  Stir in 2 cloves garlic and thyme and cook 3 minutes.  Stir in balsamic vinegar and cook 3 minutes or until onions begin to caramelize.

Add beer and bring to a boil; boil 1 minute.  Add beef broth and bring to a boil over high heat.  Reduce heat to low and simmer 20 minutes.

Meanwhile, melt remaining 2 tablespoons butter in a large skillet.  Add garlic and cook 1 minute.  Stir in bread cubes and cook, stirring frequently, until bread is golden brown.

To serve:  Ladle soup into ovenproof bowls.  Top with garlic croutons and 2 slices cheese.

Place under preheated broiler and broil 2 minutes or until cheese is melted.