Slice the bacon into lardon (small strips).

Render lardon until crispy on medium heat. Add sliced onion. Cook the onions down on medium heat until they have a deep brown colour, stirring every 2-3 minutes.

Turn down the pot to low and continue to cook for an hour or more, stirring less often, approximately each 10 minutes. The natural sugars will come out of the onion, as they become a beautiful texture. When the onions and bacon look brown and caramelized, add 4 tbsp of red wine vinegar. Cook for 5 minutes.