Combine mayonnaise, lemon juice, TABASCO® Sauce, and salt in small bowl and mix well. Stir in arugula, watercress, and green onion; mix well.
Cut 1 1/2-inch rounds from bread and spread lightly with softened butter. Spread 1/2 teaspoon arugula mixture on each bread round. Cover with plastic wrap and chill.
Makes 6 dozen.