1. In medium-size bowl, whisk together flour, sugar, salt and nutmeg. Stir in butter and sour cream until mixture forms a ball. Divide dough into thirds. Dip each third in flour to coat. Transfer dough to 3 sheets of plastic wrap; press each into flat disk. Wrap and refrigerate 2 hours or overnight.Filling:
2. Soak plums in the boiling water for 5 minutes to soften. In food processor, puree plums, 1 tablespoon soaking liquid, sugar and nutmeg.
3. Heat oven to 350 degrees F. Line baking sheet with aluminum foil.
4. Between sheets of floured plastic wrap, roll one third of dough into 10 x 8-inch rectangle, turning dough 2 or 3 times and flouring lightly as needed to prevent sticking. Remove top sheet.
5. Spread 3 tablespoons plum puree over dough, leaving 1/2-inch border along long sides. Starting from long side, roll up dough using plastic wrap as guide, to enclose filling. Place in freezer 15 minutes. Repeat with remaining dough, filling for 2 more rolls; freeze as above.
6. Cut each roll crosswise into 12 equal slices, about 3/4 inch thick. Transfer slices, seam side down, to prepared baking sheet.Glaze:
7. Brush cookies with egg white; sprinkle with sugar.
8. Bake in 350 degree F oven for 15 to 20 minutes or until tops are lightly browned. Remove cookies to wire rack; let cool. Store in airtight container at room temperature for up to 1 week. Makes 3 dozen cookies.