How to Make It
Preheat oven to 425°F. Line a baking sheet with parchment. Rub potatoes with 1/2 Tbsp. oil, then sprinkle with salt. Place potatoes on a paper towel and microwave until just tender, about 10 minutes, turning halfway through. Transfer potatoes to baking sheet. Bake until skins are crisp, about 15 minutes.
Heat remaining oil in a large skillet over medium-high heat. Cook onion and mushrooms, stirring occasionally, until soft and beginning to brown, about 5 minutes. Stir in pepperoni, tomato sauce, Italian seasoning and 1/4 tsp. each salt and pepper.
Remove potatoes from oven. Place oven rack 5 inches from heat source; preheat broiler. Carefully cut a slit in top of each potato; squeeze ends to open. Fluff inside of potato with a fork. Divide pepperoni mixture among potatoes; top with mozzarella. Broil until cheese is melted and starting to brown, approximately 2 minutes. Serve immediately.