Trim away silvery sinews from the venison fillet using a sharp knife.
Rub the meat with oil and season well with salt and pepper.
Sear the meat in a very hot frying pan for 2 minutes until golden-brown.
Reduce heat and fry for 6 to 8 minutes while turning regularly.
Rest the meat on a board under foil.
Deglaze the pan with water, scraping up browned bits, then cool for the dressing.
Boil the green beans for 5 minutes until tender, then refresh in cold water.
Whisk mustard, walnut juice, gherkin, pepper, Worcestershire sauce, and pan juices together.
Stir chopped tarragon and red wine vinegar into the dressing mixture.
Slice the venison diagonally and add any resting juices to the dressing.
Assemble salad with beans, watercress, walnuts, shallot, and venison on plates.
Drizzle with walnut oil and dressing before serving.