Place the vinegar, parsley stalks and chervil into a small saucepan and bring to the boil. Simmer until the volume is reduced to one tablespoon of liquid.
Strain the liquid through a sieve and discard the parsley and chervil.
Place the egg yolks, mustard powder and infused vinegar into a food processor and blend.
With the blender still running, gradually add the oils. Continue to blend the mixture until all the oil has been incorporated and the mixture has thickened.
Season with salt and freshly ground black pepper and add a squeeze of lemon juice.
Spoon into a small serving bowl and serve.