Adjust the oven rack to accomodate a large dutch oven or stockpot, then preheat to 300 degrees. Sprinkle meat all over with the 3 Tbs flour, salt, and pepper. On stovetop, heat oil in stockpot over high heat and brown meat all over. Add broth, mustard, and thyme, cover and place in oven. Roast 1 hour, then add onions (ok if still frozen), potatoes, and carrots, recover and continue cooking for 2 to 2 1/2 hours longer. Remove pot to stovetop, and place meat and veggies on a serving plate. Cover with foil to keep warm while you make the gravy. In a small bowl with a spoon, knead together the remaining flour with the butter, the whisk into liquid in pot. Simmer on med-high heat for about 5 min until thickened, then pour into a gravy dish to be served with the roast.