In a shallow bowl, make the marinade by combining soy sauce, brown sugar, lemon juice, olive oil, garlic, onion, ginger and pepper. Coat beef tips with marinade. Cover and refrigerate for at least 6 hrs.
Remove steak tips from marinade and pat meat dry. Reserve marinade.
Heat a large skillet over medium-high heat with oil. When oil is hot and begins to smoke, add the beef and brown 3 min. on each side. Do not overcook. Remove when evenly deep brown and reserve on a plate.
Add remaining 3 Tbsp. olive oil to skillet, heat until oil ripples then add mushrooms and brown 7-8 min. Add garlic after 5 to 6 min. Season mushrooms with salt and pepper and cook a few minutes more.
Add sherry, reduce 1 minute. Add 1 cup of reserved marinade (strained). Reduce heat and cook, reducing liquid until thickened.
Add beef tips, their juices and mushrooms to the pan, cook for 2 min. Serve with mashed potatoes and mixed vegetables on the side.