Arrange potatoes in a microwave-safe 2-quart square dish. Microwave, uncovered, on 100 percent power (high) for 7 minutes. Turn potatoes; microwave for 8 minutes more. Let potatoes rest in microwave.
Meanwhile, slice steak into thin strips, slicing crosswise against the grain. In a 12-inch skillet heat oil over medium-high heat. Add steak; cook and stir for 2 minutes. Add mushrooms and broccoli; cook and stir for 5 minutes. Add stir-fry sauce; stir to coat. Cook, uncovered, for 1 minute or until heated through.
Carefully remove potatoes from microwave. Cut a lengthwise slit in the top of each. Squeeze potato ends to open. Top potatoes with stir-fry mixture. Drizzle with any remaining sauce. If desired, top with sweet pepper slices and peanuts.