Pierce potatoes all over with a fork. Rub vegetable oil over the potatoes, and place them on rimmed baking sheet. Sprinkle with salt and pepper.
Bake potatoes for about 1 - 1 ¼ hours, or until a fork is easily inserted.
Combine all ingredients in a medium bowl. Set bowl over pot of simmering water to create a double boiler. Whisk gently until cheese melts and mixture is smooth.
Remove from heat. Makes about 1 ¼ cups. (Can be made 1 day ahead. Cover and refrigerate. Rewarm over low heat, or microwave, when ready to use.)
Heat 1 tablespoon olive oil in a large skillet over medium heat. Add onion and peppers. Season with salt and pepper.
Sauté until vegetables are soft and caramelized, stirring occasionally, 12-15 minutes.
Transfer to bowl and cover with foil. (Can be made 1 day ahead. Cover and refrigerate.)
In the same skillet, heat 1 tablespoon oil over medium-high heat. Add mushrooms and rosemary, and sauté 5 minutes, or until mushrooms are tender, stirring frequently.
Add garlic and cook 1-2 minutes, stirring frequently. Season to taste with salt and pepper.
Transfer to separate bowl and cover with foil. (Can be made 1 day ahead. Cover and refrigerate.)
Heat the last tablespoon of oil in same skillet over medium-high heat. Add shaved steak and cook until browned, stirring frequently, about 5 minutes.
When potatoes are cool enough to handle, cut off the top third and scoop the soft potato into a large bowl, leaving a ¼-inch rim throughout the potato shell.
Mash the potato with half of the steak, ½ cup whiz, and ½ cup milk. Season to taste with salt and pepper. Slightly mound the potato shells with mashed filling.
Arrange potato tops on a rimmed baking sheet. Spread a little whiz over each. Set stuffed potatoes on the potato tops. (Can be made 1 day ahead of final baking. Tent with foil and chill. Let sit at room temp for 1 hour before continuing.)
Top the potatoes with remaining steak, more whiz, and sautéed onion, peppers, and mushrooms, as desired.
Bake in 400°F oven until heated through, 10-15 minutes.
Transfer potatoes to platter and serve. Pass remaining Whiz separately. Enjoy!