Preheat oven to 325° F. Lightly grease a baking sheet.
Wipe mushrooms with a damp towel to remove any dirt from the mushrooms, do not hold or rinse under water. Remove and chop the mushroom stems. Set the caps to the side.
Melt the butter in a skillet over medium heat. Add the mushroom stems, onion, pepperoni, and garlic to the butter and cook until the onions are tender. Stir in the cracker crumbs, Parmesan cheese, oregano, salt, pepper, and chicken broth; remove from heat and allow to cool to the touch.
Stuff the mushroom caps with the pepperoni mixture. Arrange the stuffed caps on the prepared baking sheet. Bake in the preheated oven until filling is hot and mushrooms are cooked through, 25 to 30 minutes