For the mayonnaise, wash parsley, shake dry and chop the leaves finely. Blanch briefly in a little water. Drain well. Mix egg yolk and vinegar, add oil first drop by drop, then in a thin stream until a mayonnaise is formed.
Stir in the parsley. Season with salt and pepper.
For the tartar, remove the oyster meat from the shells. Peel and finely dice the shallot. Dab steak dry and cut into small cubes. Mix steak, shallot, oyster meat and oil. Season with salt and pepper.
Serve with the mayonnaise.