Trim fat from steak, and cut steak diagonally across the grain into thin slices. Combine steak and cornstarch in a small bowl, and toss well. Heat oil in a large nonstick skillet over medium-high heat. Add steak, and sauté 5 minutes. Add onion, and sauté 1 minute. Add mushrooms; cover and cook 2 minutes. Add broth, salt, and pepper. Reduce heat, and simmer, uncovered, 5 minutes. Remove from heat; stir in sour cream and parsley. Serve with rice.