1.Heat oil in large frying pan; add liver, stir over high heat about 1 minute
2.Add butter and shallot to same pan; cook, stirring, until shallot softens.
3.Add sherry to pan; simmer until liquid is reduced by half. Add stock; simmer until liquid is slightly thickened. Return liver to pan; stir until heated, season to taste.
4.Toast bread slices lightly both sides.
5.Spoon liver mixture over toast; sprinkle watercress with vinegar. Top toasts with watercress mixture.