In a medium dutch oven or stockpot, melt the butter and add the onions.
Cook the onions on a low flame until they begin to caramelize and become golden brown, approximately 45 minutes. Stir occasionally.
Once the onions are golden brown and caramelized, add the flour, the stir to combine.
Pour in the beef broth and stir. Cover the soup and let simmer on a low flame for 30 minutes.
Preheat the oven to broil, then ladle two scoops of soup into a broil-safe ramekin or bowl, then add a slice of French baguette and top with cheese. Add to the broiler for2-3 minutes or until the cheese is melty and slightly golden brown. Serve with fresh thyme.
Notes
LOW AND SLOW: Cook the onions on a low to medium-low heat for a prolonged period, from 45 minutes to sometimes closer to an hour. They should give off a sweet aroma and be golden brown, not burnt.Stir occasionally: Stir the onions every few minutes to ensure even cooking and prevent them from sticking.Add liquid for moisture: If the onions start to stick or burn, deglaze the pan with a splash of water, broth, or wine to help release the onions from the bottom of the pan. Be patient: Allow the natural sugars to caramelize, and the water content to evaporate, concentrating the onion flavor to achieve the desired deep golden-brown color and naturally sweet flavor.