Position a rack in the center of the oven and preheat to 350 degrees F. Brush a 9 by 5-inch loaf pan with butter and dust it with flour.
Whisk the flour and salt together in a bowl.
In another medium bowl, rub the citrus zests into the sugar with your fingers until the sugar is fragrant and takes on a bit of the fruit color. Beat the eggs into the sugar with a wooden spoon until blended. Stir in the flour mixture to make a smooth, sticky but pourable batter. Stir in the pistachios. Pour the batter into the prepared pan, and smooth the surface lightly with a spatula.
Bake until the loaf is lightly browned and pulling away from the sides of the pan, about 40 to 45 minutes. A skewer inserted into the center should come out clean, and the loaf will look like a not so great pound cake. Let stand for 10 minutes, then remove from the pan and cool on a rack for 1 hour.
Wrap the loaf in plastic and freeze until firm, about 30 minutes.
Preheat the oven to 300 degrees F.
Slice the chilled loaf as thin as you can -- 6 to 8 slices to the inch. (It helps to mark the loaf lightly with a serrated knife every half-inch and use that as a guide.) Lay the slices flat on 2 baking sheets and bake until brown, 10 to 15 minutes. (Take care -- the cookies can go from pale to dark brown quickly.)
Let stand overnight or for at least 12 hours on a rack to cool and crisp up. Serve. Store in a well-sealed jar or other container.
The loaf can be stored well-wrapped at room temperature for a week, or frozen for a month. The melbas are best stored in an airtight container, away from moist cookies and cakes. Freeze slices double-wrapped in plastic wrap for up to 1 month. Defrost in their wrappers.
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