1. Heat oil in large skillet over medium heat; sauté mushrooms until tender, about 3 minutes.
2. Add beef tips to skillet; cook 13 to 15 minutes, stir occasionally.
3. Meanwhile, place pierogies in boiling water and cook for 6 to 7 minutes; drain.
4. Remove beef tips from heat; stir in sour cream.
5. Serve over cooked pierogies.