Preheat your oven to 400°F. Using a fork, pierce the potatoes to release steam while they bake. Place the potatoes on a baking sheet and coat generously with olive oil. Season with salt and bake for 45 minutes to 1 hour, until the potatoes are tender. While they’re baking, prepare the steak and cheese whiz.
To prepare the steak, heat 2 tablespoons of olive oil (or render the fat from the sirloin steak) in a large sauté pan over medium-high heat. Add the onions (and green bell peppers, if using) and season with a pinch of salt. Cook for 5-7 minutes until softened and slightly charred. Remove from the pan and set aside. In the same pan, add a little more oil or fat, if needed, then add the thinly sliced sirloin steak. Season with salt and pepper, cooking for 2-3 minutes. Add Worcestershire sauce and continue cooking until the steak is fully cooked through. Taste for seasoning and remove from heat. Keep the steak in the pan to reheat before serving.
For the cheese whiz, whisk together the cornstarch, garlic powder, onion powder, and mustard powder in a small bowl. In a medium saucepan, add the milk and simmer over low-medium heat. Slowly add the dry ingredients, whisking for 1-2 minutes until the mixture thickens into a runny consistency. Lower the heat, then add the American cheese and cheddar cheese, mixing until smooth. Taste for seasoning, adding salt or more seasonings as needed. Turn off the heat and cover the pan until ready to serve.
Once the potatoes are done baking, remove them from the oven and allow them to cool slightly. Cut each potato in half lengthwise. Adjust the oven temperature to 450°F. Scoop out the inside of each potato and reserve in a medium bowl (use the potato filling for another recipe like tater tots!). Coat the inside of the potato skins with olive oil and drizzle more on top. Place the skins back on the baking sheet and bake for 8-10 minutes until they are golden and crispy. Remove from the oven and set aside.
Before serving, reheat the cheese whiz and steak. Arrange the crispy potato skins evenly on a serving board or platter, then fill them with the steak. Add the sautéed onions and green bell peppers (if using) to the board, along with the cheese whiz and halved provolone slices. Optional: For extra cheese pull, use a blowtorch on the provolone slices for a melty effect! Be careful with the torching, but it’s a fun touch if you have one!