Heat oven to 400°F. Brush large cookie sheet with 1 teaspoon of the olive oil. Unroll dough on cookie sheet; press into 13x9-inch rectangle. Brush dough with 1 teaspoon of the olive oil; sprinkle with 1/4 teaspoon salt. Bake 8 minutes.
Meanwhile, using sharp knife, thinly slice steak across the grain; cut slices into 1-inch pieces. In 12-inch nonstick skillet, heat 1 tablespoon of the olive oil over high. Add steak; cook 2 minutes, stirring occasionally. Remove from heat. Stir in cheese until melted.
Spoon steak mixture on crust; sprinkle with walnuts. Bake 7 to 9 minutes or until crust is deep golden brown.
In medium bowl, mix remaining 1 teaspoon olive oil, lemon juice, 1/8 teaspoon salt and 1/8 teaspoon pepper. Add arugula, toss to coat. Top flatbread with arugula mixture.