Trim fat from meat. Sprinkle with salt and pepper. In a 4-1/2- to 6-quart Dutch oven over medium heat brown roast on all sides in hot oil. Remove meat. In same pan, cook onions until browned. Remove onions. Carefully wipe excess oil from pan. Return meat and onions to pan. Combine the 3/4 cup water, the vinegar, thyme, and 1/4 teaspoon salt. Pour over roast and onions. Bring to boiling; reduce heat. Simmer, covered, for 1 hour.
Meanwhile, if using new potatoes, peel a strip of skin from the center of each. If using medium potatoes or sweet potatoes, peel and cut into eighths. Add potatoes, carrots, and celery to meat. Return to boiling; reduce heat. Simmer, covered, for 45 to 55 minutes more or until meat and vegetables are tender. Transfer meat and vegetables to a serving platter, reserving juices in Dutch oven. Keep warm.
For gravy, measure juices; skim off fat. If necessary, add enough water to juices to equal 1-1/2 cups. Return to Dutch oven.
In a small bowl whisk together the 1/2 cup cold water and flour until smooth. Stir into juices in pan. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. If desired, season with pepper. Serve gravy with meat and vegetables. Makes 8 to 10 servings.