Cut beef crosswise (against the grain) into very thin slices, sprinkle with ¼ teaspoon each salt and pepper.
In 12 inch nonstick skillet over medium high heat, heat 2 tablespoon oil, add beef cook and stir 2 t 3 minutes until browned.
Using slotted spoon, remove beef to plate. In same skillet over medium high heat, heat remaining 1 teaspoon oil, add garlic, cook and stir 30 to 60 seconds until fragrant.
Add frozen vegetables, ¼ cup water and remaining ¼ teaspoon each salt and pepper, cook, covered 3 minutes until tender.
Meanwhile, in small bowl stir peanut butter and remaining ½ cup water to blend; add to skillet along with beef and any accumulated juices.
Cook stirring 1 to 2 minutes longer until heated through and coated with sauce.
