In a Dutch oven or deep large heavy skillet, heat oil over moderate heat. Add roast and brown all sides. Season with salt and pepper to taste.
In a medium bowl, combine garlic, onion, soup, ½ cup water, brown sugar, vinegar, dry mustard, and Worcestershire sauce, blending well. Spoon soup mixture over roast. Reduce heat, cover, and simmer for 2½ to 3 hours or until meat is fork tender. Transfer meat to a warm platter, cover, and keep warm while preparing gravy.
To prepare gravy, measure pan liquid; add enough water to make 2 cups and return to pan. In a 1-cup measure, blend a small amount of cold water into the flour, blending until smooth. With a wire whisk, blend flour mixture into pan liquid. Cook, stirring constantly, over moderate heat until mixture is bubbly hot and slightly thickened. Adjust seasoning to taste. May add 2 to 3 drops gravy flavoring and seasoning sauce, if desired.
To serve, cut meat across the grain into thin slices. May pour gravy over roast or pass in a gravy boat. Garnish roast with parsley sprigs or mince parsley.
Variations: Add 8 ounces sliced fresh mushrooms to soup mixture during last 20 to 30 minutes of cooking.
Add about 16 peeled whole very small carrots to soup mixture during the last 30 to 40 minutes of cooking.
Add 1/3 cup drained diced pimento to soup mixture during cooking.