Preheat the oven to 475 degrees F. Set roast in a pan. Brush with mustard; sprinkle with thyme, salt and pepper. Roast 10 minutes. Reduce the heat to 325 degrees F and roast to the desired doneness. The best way to gauge that is to use a meat thermometer. For rare beef, the centre of the roast should register 125 to 130 degrees F; for medium rare, 130-135 degrees F; and for medium, 140 degrees F.
While the roast cooks, melt the butter in a skillet set over medium-high heat. Add mushrooms and shallots and cook until tender. Mix in flour, tomato paste, garlic and tarragon and cook 2 minutes more. Slowly, stirring steadily, mix in the stock. Simmer sauce until lightly thickened, about 5 minutes. Remove from the heat.
When cooked, transfer the roast to a plate, tent with foil and rest for 10 minutes. Set the pan over medium-high heat. Add the wine and simmer until reduced by half. Pour in the mushroom sauce and return to a simmer; season with salt and pepper. Slice and plate the beef; serve the sauce in a sauceboat.