Pick off any hairy stems from the watercress and coarsely chop the remaining stems and leaves. Chop the shallot.
Heat the vermouth and shallot in a small saucepan until just boiling, then add the demi-glace and cream. Stir in the watercress.
Add a little lemon juice and taste to check the balance of the seasoning. Bring the sauce to the boil, then serve.