Preheat oven to 425°F.
Line a baking sheet with parchment paper.
Rub potatoes with ½ 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 the baking sheet. Bake until skins are crisp, about 15 minutes.
Heat remaining oil in a large skillet over medium-high heat.
Cook onions and mushrooms, stirring occassionally, until soft and beginning to brown, about 5 minutes.
Stir in pepperoni, tomato sauce, Italian seasoning, and ¼ tsp. each salt and pepper.
Remove potatoes from the oven.
Place oven rack 5 inches from the heat source; preheat boiler.
Carefully cut a slit in the top of each potato; squeeze ends to open. Fluff inside of the 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.
