Preheat an oven to 180 degrees C. Lightly grease a 20x30cm baking dish.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the shell pasta and return to a boil. Cook the pasta uncovered, stirring occasionally until the pasta has cooked through but is still firm to the bite, about 13 minutes. Drain well in a colander set in the sink.
Heat a large frypan over medium-high heat. Cook and stir beef mince, onion and green capsicum until browned. Drain off excess grease. Stir in pepperoni, crushed tomatoes, tomato juice, oregano, celery salt, onion powder, garlic salt, basil, chilli flakes and cooked pasta. Pour into the prepared baking dish and cover with aluminium foil.
Bake in the preheated oven for 20 minutes. Remove casserole from oven and sprinkle with mozzarella cheese. Return to the oven and bake until the cheese melts, about 5 minutes.