Preheat oven to 425F. Prep a baking dish by lightly greasing.
Trim ends from zucchini and cut each in half crosswise to make four portions.
Make ¼ to ½ inch slices crosswise in each portion of zucchini, being careful not to cut all the way through. Stop about ¼ inch from the bottom keep the slices connected along the edge.
Carefully transfer zucchini to baking dish. Gently fan open to separate the slices more.
Combine olive oil, Italian seasoning, and salt & pepper to taste. Brush the zucchini (inside and out) with the oil.
Fill the slices with mozzarella and pepperoni. Depending on the size of your slices, you can alternate fillings or put both pepperoni and mozzarella in each.
Bake uncovered for about 15 minutes, or until cheese has begun to melt and pepperoni/zucchini is lightly browned around the edges.
In the meantime, combine garlic parmesan topping ingredients.
Top zucchini with sauce (about 2 tablespoons each) and sprinkle with parmesan mixture. Return to oven for 8-10 minutes, or until zucchini is fork tender.
Optional: Top with shredded mozzarella and broil for 2-3 minutes, until melted and lightly browned.
For best results, tent with foil and let rest for about 5 minutes before serving.