Preheat your oven to 375°F (190°C).
In a skillet, brown the lean ground Italian sausage until it's cooked through. Drain any excess fat, then set it aside to cool slightly.
Slice each bell pepper in half lengthwise and gently scoop out the seeds and white ribs. Arrange these pepper halves snugly in a 9x13-inch baking dish (a quick spray with nonstick cooking spray helps prevent sticking!).
In a large mixing bowl, combine the cooled sausage, marinara sauce, most of the diced pepperoni, half of the mozzarella cheese, and the chopped Kalamata olives. Give everything a good stir until it's all nicely mixed.
Generously spoon this savory pizza filling into each bell pepper half, making sure they're packed full of flavor.
Cover the baking dish tightly with aluminum foil and bake for 15 minutes.
Carefully remove the foil. Sprinkle the remaining mozzarella cheese and the rest of your diced pepperoni over the top of each stuffed pepper.
Pop it back into the oven, uncovered, for another 15 minutes, or until the cheese is beautifully melted, bubbly, and golden brown.