Heat oven to 400 degrees. Rub a very thin layer of oil over the baked potatoes & sprinkle with salt. Bake on a baking sheet for 50-60 minutes, turning every 20 minutes, until done. Let cool. Slice in half, scoop out the potato filling - leaving a 1/4" of potato on the skin so that the potato skin is still sturdy. Place the scooped out potato in a large bowl & set aside.
Meanwhile, heat a little olive oil in a large skillet. Add the onions & season with salt and pepper to taste. Cook for about 4 minutes. Add the sliced mushroom, and stir. Cook for an additional 6 minutes, or until the veggies are at the texture you want. Add in the minced garlic & cook for another minute, stirring. Mix the onion/mushroom mixture into the mashed potatoes along with the 10 slices of chopped deli roast beef. Stir in the cheese (or nutritional yeast if you're doing dairy free).
Scoop the filling into the empty potato skins. Bake on a baking sheet at 400 degrees for 15 minutes. Sprinkle with an additional 1/2 cup of cheese (if desired) and return to the oven for a couple minutes, or until the cheese is melted.