6 large yellow onions (if onions are really large I cut the amount to 4 instead of 6)
3 Tablespoons olive oil
One 14-ounce can chicken broth
One 10.5 ounce can beef broth
2 tablespoons Marsala or red wine (optional)
8 ounce Gruyere cheese, cut into slices (I used Swiss cheese)

Peel and thinly slice onions.
Put onions in the slow cooker and toss with the olive oil.
Cover and cook on LOW for 3 to 4 hours, stir and continue cooking several more hours or until onions are well cooked and carmalized.
Add the broth and wine (if using); add no water or salt.
Cover and cook on HIGH until hot, 15 to 30 min.

Add toasted bread slices and a slice of the cheese if desired to individual bowls place under the broiler, about 4 inches under the flame and broil until cheese is melted.

I am lazy so I just put the toasted bread and cheese on top of the individual bowls and nuke it until the cheese melts!