To prepare filling, cook the onion and turkey in a large nonstick skillet coated with cooking spray over medium-high heat until turkey is browned, stirring to crumble. Add cabbage; cook until cabbage wilts, stirring constantly. Stir in pepper and 1/4 teaspoon salt. Cover and chill.
To prepare dough, dissolve sugar and yeast in warm water in a large bowl; let stand 5 minutes. Stir in milk, oil, 3/4 teaspoon salt, and eggs. Lightly spoon flour into dry measuring cups; level with a knife. Add 3 1/2 cups flour to yeast mixture, and stir to form a soft dough. Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 8 minutes); add enough of the remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down; cover and let rest 5 minutes.
Divide dough in half. Roll each half into a 10 1/2 x 7-inch rectangle on a lightly floured surface. Cut each rectangle into 6 (3 1/2-inch) squares. Working with 1 portion at a time (cover remaining dough to keep from drying), spoon about 1/4 cup filling into center of each portion, and bring 2 opposite corners to center, pinching points to seal. Bring remaining 2 corners to center, pinching points to seal. Pinch 4 edges together to seal. Place the bierocks, seam sides down, on a large baking sheet covered with parchment paper. Cover and let rise for 20 minutes.
Preheat oven to 375°.
Uncover bierocks. Bake at 375° for 15 minutes or until bierocks are browned on the bottom and sound hollow when tapped. Remove bierocks from pan, and cool on wire racks.
Note: To freeze, cool completely, and wrap individually in foil. Place wrapped bierocks in a heavy-duty zip-top plastic bag; freeze for up to 3 months. To reheat, thaw in the refrigerator. Place foil-wrapped bierocks in a preheated 350° oven for 15 minutes.