Preheat oven to 400 degrees. Wash potatoes and thoroughly dry. Pierce potatoes several times with a sharp knife and place on a baking sheet. Using your hands, rub 1 Tbsp of olive oil over the outsides of the potatoes. Sprinkle the potatoes lightly with kosher salt. Bake in the preheated oven for 50 minutes. Remove from the oven and let cool for 10 minutes or until cool enough to handle.

In a skillet heat the remaining 1 Tbsp of olive oil over medium-high heat. Cook the mushrooms for 3 minutes until beginning to brown. Add the onions and bell peppers to the pan and season with a pinch of salt and pepper. Cook for 3 more minutes, stirring frequently. Add the roast beef to the skillet and cook for 2-3 minutes, or until the meat is cooked to your liking.

Slice the potatoes in half lengthwise and scoop out the flesh leaving abut 1/4 inch of the potato. Brush the inside of the potato with the melted butter, then flip face down and return to the oven for 5 minutes. Flip the skins using tongs and cook for an additional 3 minutes.

Sprinkle a little bit of shredded cheese into each skin, fill with the meat mixture, then top with remaining shredded cheese. Return to the oven and cook for 4 minutes, or until the cheese is melted. Top with green onions, if desired.