In a large bowl, whisk together vinegar, mustard, and oil; season with salt and pepper. Set dressing aside.
Season the steak generously on both sides with salt and pepper, and place in a shallow baking dish. Drizzle with 2 tablespoons dressing and turn to coat; leave remaining dressing in bowl. Let steak marinate 5 to 10 minutes.
In a large nonstick skillet, cook the steak over medium-high heat until medium-rare, 5 to 8 minutes per side. Transfer to a plate, cover loosely, and let rest 10 minutes. Then transfer to a cutting board, and slice thinly, across the grain.
To bowl with remaining dressing, add watercress and orange slices; toss to coat. Divide salad evenly among plates; top with steak slices, and sprinkle with cashews. Serve immediately.