Preheat the oven to 425 degrees F. Cut the lamb racks into 4, 4-rib racks and set in a roasting pan. Brush the top and sides of the lamb with the mustard; sprinkle with herbs de Provence, salt and pepper. Roast the lamb 22-25 minutes for rare, 25-28 for medium rare and 30 for medium. When the lamb is just about ready, fill a large skillet with a 1/4-inch of water and bring to a boil. Add the spinach and cook until just wilted, reserve on low heat. When done, set the lamb on a plate, tent with foil, and rest 10 minutes. Place the roasting pan on the stovetop over medium-high. Add the port and reduce by half. Whisk the flour and stock in a bowl; add to the pan. Simmer 5 minutes. Divide and place the spinach, draining away the excess moisture first, on 4 heated plates. Slice the lamb racks and divide and set on top of the spinach. Drizzle with some of the sauce and serve. Serve the rest of the sauce in a sauceboat.