3 medium-sized onions
4 tbsp olive oil
2 tbsp butter
1 tbsp balsamic vinegar
1/2 tbsp black pepper
Proceed as follows:
Cut the onions in very thin slices but do not chop them
Heat extra virgin olive oil and butter in a pan, add the onion slices and coarsely ground black pepper and let simmer at very low heat for 45 minutes, while occasionally stirring.
Add balsamic vinegar and continue the simmering for another 5-10 minutes.
Let the paste cool overnight in the refrigerator.
The paste can best be served as a condiment to, e.g. plaice or paté.