Clean the beef tongue, scrubbing the surface. Cut it nto 12 slices and lay them flat them into cooking pot. Pour 1/2 of the beef broth over the beef tongue slices and cook slowly over small heat for about 2-2.5 hours.
Cut bacon into cubes. Melt butter in a second cooking pot and fry bacon about 2 min. Add shallots and fry 5 minutes over high heat, stirring constantly. Add flour and keep stirring until mixture turns brown. add the red wine and cook for another 5 minutes, stirring occasionally. Add the remaining beef broth and season to taste with salt, sugar and mustard.
Lift beef tongue slices out of the cooking pan, place into a serving bowl and serve, covered with the red wine sauce, together with mixed vegetables (Leipziger Allerlei) and spaetzle, dumplings or mashed potatoes.