Melt butter in a large pot on low heat. Add onions, thinly sliced, with a pinch of salt. Cook for 2 hours, stirring regularly, until deep, golden brown.
Next, stir in the flour and thyme leaves, cook for 2 minutes. Then stir in the vinegar with wine, whisk in the beef stock. Bring to boil. Turn the heat down and simmer for 1hour.
Once the sup is almost ready, prepare your cheesy baguette croutons. Preheat the oven to 190C. Slice the bread and cover with grated gruyere. Place on the baking tray and bake for a few minutes until the cheese is melted.
Serve in deep bowls. Place 2 croutons on top and enjoy zis French classic.