Whisk peanut butter, orange juice, soy sauce, vinegar and sugar in a medium bowl until smooth.

Heat 2 teaspoons oil in a wok or large skillet over medium-high heat. Add garlic and cook, stirring, until fragrant, 30 seconds. Add steak and cook, stirring, until browned and barely pink in the middle, 2 to 4 minutes. Transfer to a bowl.

Reduce heat to medium. Swirl in the remaining 2 teaspoons oil. Add cabbage and 2 tablespoons water; cook, stirring, until beginning to wilt, 3 to 5 minutes. Add carrots (and more water if necessary to prevent sticking or burning) and cook, stirring, until just tender, about 3 minutes more. Return the steak and any accumulated juices to the pan, then pour in the peanut sauce and toss to combine. Serve sprinkled with peanuts (if using).

Tip: For thinly sliced beef, freeze for 30 minutes to make it easier to cut into very thin slices.