Wipe lamb with damp paper towels. Trim excess fat. Spread flat on board, pound with mallet to make even thickness. Pour lemon juice over lamb to cover completely.
For filling: In bowl, combine chopped onion, parsley, salt, basil, marjoram and garlic; mix well. Spread onion-parsley mixture evenly over lamb to within 1 inch of edge all around. Starting at short side, roll up, tie roll with kitchen twine at 2-inch intervals to secure. If necessary, close ends with toothpicks.
Slowly heat the butter in an 8-quart Dutch oven. In hot butter, brown roast evenly on all sides, turning with wooden spoons - takes about 25 minutes. Spoon off excess fat. Pour beef broth into a 2-cup measure. Add water to measure 1-1/2 cups. Add to lamb along with bay leaf, bring to the boiling point. Reduce heat, simmer, covered, 1-1/2 hours, turning meat at least once.
Wash potatoes; pare strip around each one. Add potatoes and onions to Dutch oven. Simmer, covered, 40 minutes, or until lamb and vegetables are tender (test vegetables with a fork).
Remove lamb, potatoes and onions to serving platter. Keep Warm. Remove string from lamb, let stand 20 minutes for easier carving.
Skim fat from pan liquid. Measure liquid; add water to make 1-3/4 cups. Mix flour with 1/4 cup cold water until smooth.
Stir into pan liquid, bring to boiling, stirring. Add chopped mint. Reduce heat and simmer 3 minutes. Spoon some of mint gravy over meat, and pass the rest. Garnish serving platter with lemon slices and fresh mint leaves, if desired.