Place potatoes into a pot, and fill with enough water to cover. Bring to a boil and cook until tender but still firm, about 10 minutes. Drain and cover to keep warm.
Preheat oven to broil or preheat a grill for medium-high heat. Season both sides of steak with salt and pepper. Broil or grill steaks for 6 to 8 minutes per side, or to desired doneness.
Meanwhile, whisk together oil, vinegar, mustard, and tarragon for dressing; season dressing to taste with salt and pepper.
Divide greens, tomatoes, and shallots among 4 plates. Cut warm, unpeeled potatoes into quarters; slice steak into 1/4 inch thick strips. Top salad greens with steak and potatoes; drizzle dressing over salads. Serve warm.