In a skillet, saute the onion, red pepper and mushrooms in oil until tender. Remove from the heat and stir in the spinach, cheeses, seasonings and egg white; set aside. On a lightly floured surface, roll each ball of dough into a 5-in. circle. Spoon a fourth of vegetable mixture onto the center of each circle to within 1 in. of edge. Fold dough over filling; pinch edged to seal.
Place on a greased baking sheet. Place a fresh dill sprig on each if desired. Beat egg yolk and brush over tops. Bake at 375° for 18-20 minutes or until golden brown.
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