Press tomatoes through a sieve. Set tomato purée aside.
Sauté onion in butter in a heavy skillet over low heat until tender. Add meat, salt, pepper, and bay leaf; cook over medium heat until meat is browned, stirring to crumble. Drain off pan drippings. Add tomato purée to meat mixture; cook, uncovered, over low heat 35 minutes, Remove from heat, and discard bay leaf.
Roll pastry to 1/8-inch thickness on a sheet of plastic wrap; cut with a 3 1/2-inch biscuit cutter. Place 1 teaspoon meat mixture in center of each circle. Moisten edges of pastry with water. Fold pastry in half. Press edges together with fork tines to seal. Place turnovers on a baking sheet; cover and refrigerate 2 hours.
Heat 1 1/2 inches of oil to 375° in a large skillet. Cook several turnovers at a time 4 minutes or until golden brown. Drain turnovers on paper towels. Serve hot.