Wipe lamb with damp paper towels. Trim excess fat. Spread flat on board; pound with mallet on board to make even thickness. Pour lemon juice over lamb to cover completely. 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 and string 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 mint gravy over meat, and pass the rest. Garnish serving platter with lemon slices and fresh mint leaves, if desired.