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.