Drain kraut, reserving 1/2 cup juice.
Mix flour, sage, and reserved juice; stir into the drained kraut.
Heat and stir till mixture thickens.
At 1/2-inch intervals, cut slits across franks, going almost but not quite through.
Broil franks over hot coals until hot through —they'll curl as they cook.
Place franks on bread; fill center with hot kraut; top with mustard.