Make a hearty main course of this vegetarian dish by adding crumbled feta or shaved Parmesan. Steam the beans in the microwave or on the stove while you prep the other ingredients.
Place arugula on a large platter and set aside. Heat oil in a wok (or 12-inch nonstick skillet) over medium-high heat. Add onion and olives; stir-fry until onions are soft, about 4 minutes. Add tomatoes, string beans, and thyme; cook, stirring, 2 minutes until hot (tomatoes should remain firm). Remove from heat and season with salt and pepper. Top arugula with cooked vegetables.
Place balsamic vinegar in a small saucepan over medium-high heat; simmer until reduced by half. Drizzle on top of salad and serve.