Preheat oven to 375°F (190°C).
In a large mixing bowl, combine flour and salt. Cut in butter until mixture resembles coarse crumbs.
Gradually add cold water and mix until dough forms. Wrap dough and chill for 15 minutes.
In a skillet over medium heat, cook ground beef with diced onion and minced garlic until beef is browned and onions are soft. Drain excess fat.
Stir in tomato sauce, Italian seasoning, salt, and black pepper. Simmer for 5 minutes, then remove from heat.
Roll out dough on a floured surface to about 1/8 inch thickness. Cut into 4 equal rectangles.
Divide beef mixture evenly among rectangles, top each with shredded mozzarella cheese.
Fold dough over filling and seal edges by pressing with a fork.
Brush tops with beaten egg wash for a golden finish.
Place hot pockets on a baking sheet lined with parchment paper.
Bake for 18-20 minutes until crust is golden brown and crispy.
Remove from oven and let cool slightly before serving.