With a mandolin, thinly slice the onions.
In a dutch oven or large pot, melt 2 tbsp butter.
Once the butter is melted, add the onions and stir to coat. Sprinkle salt.
Continue to work the onions until caramelized, adding butter as necessary. You will want to make sure the pan has some moisture through it.
When the onions are a deep golden, caramel color, add the beef broth and wine to the pot. Stir.
Bring to a simmer and simmer for 30 minutes.
Meanwhile, toast baguette slices in oven, toaster or pan.
Taste soup for salt and pepper, adjust to your preference.
Place crocks on baking sheet; place sliced baguette in the crocks.
Pour soup evenly in the crocks until full.
Top each crock with grated gruyere.
Bake for 10 minutes or until cheese has some golden brown.
Top with sprigs of thyme and enjoy hot!