Preheat oven to 375°F. Bake potatoes for 1 to 1-1/2 hours or until fork-tender. Leave oven on.
Meanwhile, sauté bacon in medium-size skillet over low heat until almost crisp. Add mushrooms and green onion; cook 4 to 5 minutes or until mushrooms are lightly cooked. Transfer mixture to a bowl.
Cut potatoes in half; scoop out flesh, leaving 1/2 inch on the skin. Put potato flesh in large bowl; mash until smooth. Stir in mushroom mixture, Cheddar cheese, pepper, salt and butter. Spoon back into potato skins. Place on baking sheet.
Bake in preheated 375°F oven for 15 minutes or until potato is piping hot. Top each potato with sour cream and serve.