In a small bowl, sprinkle 1/4 cup chicken stock with the gelatin; set aside 5 minutes. Cook in a microwave until liquefied, about 1 minute; set aside.
In a small saucepan, melt the butter over medium-low heat; stir in the flour. Cook 2 minutes and whisk in the remaining stock, gelatin mixture, and 1/2 cup cream. Season with nutmeg, salt, and pepper; cook 5 minutes.
In a food processor, puree the mortadella until smooth and blend in the cream mixture. Set aside to cool.
Meanwhile, beat the remaining 1 cup heavy cream until stiff peaks form; fold into the mortadella mixture. Transfer the mousse to a 2-quart dish; cover and refrigerate at least 6 hours.
Serve the mousse on toasted bread slices; top with pistachios and arugula. Drizzle with balsamic vinegar before serving.
This recipe was provided by professional chefs and has been scaled down from a bulk recipe provided by a restaurant. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.