Place the top oven rack in the center position and pre-heat the oven to 350°F. Pour half the tomato sauce into a medium baking dish and spread it into a thin layer. Set aside.
Melt the butter in a large skillet over medium heat. Add the mushrooms and sprinkle with garlic powder and oregano. Season with salt and black pepper, to taste, and stir to combine. Cook, stirring occasionally until the mushrooms develop some color and release their liquids, approximately5 minutes.
Remove from heat and stir in the remaining tomato sauce, pepperoni, and one cup Mozzarella cheese. Carefully spoon the pepperoni-mushroom mixture into the Poblano pepper halves and arrange them in the baking dish once filled.
Divide the remaining Mozzarella cheese and Parmesan cheese on top of the stuffed peppers and sprinkle the remaining oregano on top.
Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the peppers are fork-tender and the cheese topping is nicely browned. Remove from oven and cool for several minutes before serving. Enjoy!