Set your crock pot on high, then add onions, garlic, brown sugar, butter, salt and balsamic and mix until combined. Cover and let cook for 60 minutes, stirring occasionally, until onions are a bit caramely and brown on the edges. Add in flour, then stir thoroughly and let sit for 5 minutes. Add in beer, beef stock, thyme, and pepper, then turn heat down to low, cover and cook for 6-8 hours.
Before serving, cut french bread or baguette into slices. Fill soup bowls to the top, then cover with slice of bread and a slice of cheese. Set under the broiler for 2-3 minutes, or until cheese is bubbly and golden brown. Be careful when serving as bowls will be hot!
Note: I used a very low-sodium beef stock, so depending on the stock you use, add additional salt for flavor if desired.
makes a giant pot, about 8-10 servings