Thinly slice the shallot. Melt the butter in a large pan over a low/medium heat and add the shallot, sweating for a minute or two until tender.
Step 2
Add the mustard and vinegar and stir well to combine.
Step 3
Add the stock and bring to a simmer, for a minute or so to further soften the shallot.
Step 4
Add the watercress and continue to simmer until just wilted. Season if required.
Step 5
Remove from the heat and transfer to a blender or use a stick blender to process until smooth. Add the creme fraiche and blend again until smooth and creamy to serve.