Cook all the court bouillon ingredients together for 30 minutes. Strain into a clean pan.
Poach the brain for about 10 minutes until firm to the touch. Leave to cool in the liquid, then drain.
To make the sauce ravigote, mix together the mustard, vinegar and salt and pepper until well blended. Add the capers, herbs and onion. Whisk in the oil gently as if you were making mayonnaise. (If the sauce separates, briskly whisk in a tablespoon of boiling water.)
Slice the brain thinly and arrange in one layer on a large plate. Gently spoon the sauce over the surface of the brain and if liked, garnish with sprigs of tarragon and leaves of parsley.