In a large skillet, cook the ground beef and diced onion over medium heat until the beef is browned and the onion is translucent. Drain any excess fat.
Stir in the pizza sauce, Italian seasoning, garlic powder, salt, and black pepper. Simmer for 5 minutes.
Unroll the crescent roll dough and press it into the bottom and up the sides of a greased 9-inch pie dish to form a crust.
Spoon the beef mixture into the pie crust and spread it evenly.
Top with shredded mozzarella cheese, sliced pepperoni, black olives, and bell peppers.
Bake for 20-25 minutes, or until the crust is golden brown and the cheese is melted and bubbly.
Allow the pie to cool for a few minutes before slicing and serving.