Chop the onion and cook in the bowl with butter for 2 to 3 minutes on 1000 W.
Add flour, mix, and cook again for 1 minute on 1000 W.
Add stock, vinegar, and white pepper. Cook uncovered at 1000 for 3 minutes.
Remove from the microwave and add a dash of cognac, a knob of butter (extra), and crème fraiche.
Mix well and add salt before serving.