Preheat oven to 400°F (200°C).
In a mixing bowl, combine flour and salt. Cut in butter until mixture resembles coarse crumbs.
Gradually add cold water, mixing until dough forms. Knead lightly, shape into a ball, wrap in plastic, and chill for 15 minutes.
In a skillet over medium heat, cook ground beef, onion, and garlic until beef is browned and onions are soft. Drain excess fat.
Add tomato sauce, salt, pepper, oregano, and basil to the beef mixture. Simmer for 5 minutes, then remove from heat.
Roll dough out on a floured surface to about 1/8 inch thickness. Cut into 8 equal rectangles.
Place a spoonful of the beef mixture and some mozzarella cheese on one half of each rectangle, leaving a border.
Fold dough over filling and seal edges by pressing with a fork.
Brush each pocket with egg wash for a golden finish.
Place pockets on a baking sheet lined with parchment paper.
Bake for 25-30 minutes or until crust is golden brown and crispy.
Remove from oven and let cool slightly before serving.