Place potatoes on a microwave-safe plate and prick skins with a fork. Cook in the microwave until slightly tender, 7 to 8 minutes. Cut into 1-inch cubes.

Combine potatoes, roast beef, onion, green bell pepper, and mushrooms in a bowl.

Preheat an electric skillet to 350 degrees F (175 degrees C). Coat with oil.

Spread potato mixture evenly in the skillet. Let cook, without stirring, until browned, about 5 minutes. Flip and continue cooking until second side is crisp, about 5 minutes more.