Cook pasta shells in a pot of boiling salted water for 8 to 10 minutes, until al dente, drain and rinse with cold water.
Preheat oven to 375 degrees F (190 degrees C).
Lightly grease a medium baking dish and cover with a light layer of pasta sauce.
Place the beef in a skillet over medium heat, and mix in garlic, garlic salt, and Italian Seasoning.
Cook until beef is evenly brown.
Turn off heat and mix in sour cream and 1/2 of the mozzarella.
Stuff the cooked pasta shells with the beef mixture, and arrange in the prepared baking dish.
Cover evenly with the remaining sauce.
Bake 15 minutes in the preheated oven, or until bubbly.
Sprinkle with shredded mozzarella cheese for the last 5 minutes of cooking.