Preheat oven to 350 degrees F. Prepare an 11 by 7 by 1 1/2-inch baking pan by lining with parchment paper.
Combine the sugar and cardamom in a small bowl, and mix with a spoon to combine. Beat the egg whites in a large bowl with an electric mixer on high speed until stiff, beat in sugar mixture (1 tablespoon at a time) until egg whites are thick and glossy. Add the flour, salt and pistachios, and mix with a spatula to combine. Spread mixture into the prepared pan. Bake in oven for 30 minutes, until firm to touch and top is golden. Cool for 10 minutes on a rack. Reduce oven temperature to 275 degrees F. With an electric knife, or very sharp knife, cut thin slices (1/4-inch thick, 7 inches long) and place on a large cookie sheet. Bake 20 to 25 minutes longer, or until crisp, dry, and firm. Store in airtight container.