Toast walnuts in shallow baking pan in preheated 300°F oven about 15 minutes, stirring occasionally, until aromatic. While nuts are toasting, continue as below.
Melt 1/4 cup butter in small skillet over low heat. Stir in chili oil, orange zest, cayenne and garlic salt. Cook gently about 3 minutes, stirring to combine. Pour mixture over hot nuts and stir to coat. Return nuts to oven 20 minutes more, stirring occasionally. Remove nuts with skimmer and transfer to paper toweling to drain 10 minutes. Transfer to clean paper toweling and drain 10 minutes more.
In food processor, chop nuts coarsely. Remove 1 cup and set aside. Process nuts remaining in food processor in on-off flicks until finely chopped; transfer to shallow dish and set aside for finishing sandwiches.
In same food processor, place softened butter, cheese and Armagnac. Process to combine. Add reserved 1 cup coarsely chopped walnuts and process until finely chopped but not pureed. You will have 1 1/2 cups walnut-cheese mixture.
To assemble sandwiches: Spread very thin layer of walnut-cheese mixture on 6 slices of bread. Cover with layer of beef and then a layer of watercress leaves. Place another slice of bread on top, spread with very thin layer of walnut-cheese mixture, cover with layer of beef and layer of watercress leaves. Top each sandwich with a third slice of bread and press down firmly. You will have 6 whole sandwiches.
With long, sharp knife, trim off crusts. Cut each whole sandwich diagonally into quarters, thus making triangles.
Spread edges of each triangle very lightly with remaining walnut-chesse mixture, then dip sides into reserved finely ground walnuts to finish. Cover tightly and refrigerate until serving.