Sprinkle lamb with salt and pepper; dredge in 1/4 cup flour. Place lamb, fat side up, in a large Dutch oven. Add 1 1/2 cups boiling water. Cover and bake at 350° for 1 1/2 hours or until lamb is tender, basting occasionally with pan liquid. Transfer lamb to a warm serving platter, reserving pan liquid; keep warm.
Dissolve bouillon cube in remaining 1 cup water in a small mixing bowl; add chopped mint, vinegar, and reserved pan liquid, mixing well. Set aside.
Melt butter in a saucepan; cook until golden brown. Add remaining 1/4 cup flour, stirring until smooth. Cook over medium heat 1 minute, stirring constantly. Gradually add mint mixture. Cook over medium heat, stirring constantly, until thickened and bubbly. Strain. Serve sauce with lamb. Garnish with fresh mint leaves, if desired.