lentils with sweet onions

Here's a recipe I remember from an old boyfriend of mine who was czechoslovakian.
Here's how it goes:
You'll need to cut up a large onion and fry it slowly on medium low heat until the onion is caramelized. It will take a while so be patient.
Once the onions are nice an brown, add a heaping tablespoon of white flour to your frying pan, coat all the onion pieces.
Then add your can of lentils, season heavily with salt and pepper and put a couple tablespoonfuls of white vinegar.
Voila.
Serve wish a side of sausage, or with toasts.
It's so simple but so nutritious too.