Preheat oven to 350 degrees Fahrenheit.
Heat oil in a large skillet over medium-high heat. Add veal, breaking it up with a wooden spoon; then onions, salt, pepper, and garlic powder; sauté until meat is browned.
Stir in bread crumbs, egg, and spinach; remove from heat.
Roll out one pastry sheet on a lightly floured surface. Spread half of meat mixture on pastry, leaving a 1/2-inch border on all sides. Roll up jelly roll style. Repeat with remaining ingredients.
Place each roll, seam-side down, into a lightly greased baking dish. Brush with yolk and sprinkle with sesame seeds.
Bake for 45 minutes to one hour or until veal is cooked through and pastry is golden brown.