Heat a large pot over medium heat and melt the butter.

Add the sliced onions and stir. Let them caramelize slowly for 1 ½ to 2 hours, stirring every 10 minutes.

During the last 30 minutes, stir every 5 minutes. Lower the heat if they start to brown too fast. The onions should be a deep amber color when ready.

Stir in the flour and cook for 1 minute.

Slowly add 3 cups of water and stir.

Dissolve the beef base in 1 cup of hot water, then pour it into the pot.

Add thyme, bay leaf, Worcestershire sauce, and red wine vinegar.

Bring to a simmer, reduce heat to medium-low, cover, and cook for 20 minutes.

Remove the bay leaf. Serve hot with shredded Gruyère cheese and croutons on top.