Preheat the oven to 350 degrees. Season the roast with salt and pepper. Add the olive oil to an ovenproof pan and bring to a high heat. Brown the roast on all sides, then remove it from the pan. Add the vermouth and stir to loosen all brown bits on the bottom of the pan. Return the roast to the pan and arrange the carrots, celery and onion around the roast. Roast for about 30 minutes (until the temperature reaches 115 degrees for rare or 125 degrees for medium). Transfer the roast to a large serving platter and let it rest to a few minutes.
Remove the vegetables from the pan and skim the fat from the pan juices. Bring the pan juices to a boil and add the flour and water mixture, stirring until the gravy thickens slightly, about 30 seconds. Keep warm until ready to serve.
To serve, slice the roast in thin slices and place on a platter surrounded with the carrots, celery and onions. Pour the sauce over the top.