Combine first 3 ingredients in a bowl; toss well, and set aside.
Place 2 bread slices on work surface with flat, bottom crusts together and overlapping slightly. Flatten bread slices to 1/8-inch thickness, using a rolling pin, seaming 2 bread slices together (cover bread to keep from drying). Repeat procedure 3 times with remaining 6 bread slices.
Divide corned beef evenly among bread. Divide cheese slices evenly over corned beef, and spread cabbage mixture evenly over cheese slices. Beginning with 1 long edge, roll up each sandwich, jelly-roll fashion, so seam of bread is in the middle of each sandwich (secure with wooden picks, if necessary).
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Place 2 sandwiches in skillet; cook until lightly browned on all sides, gently turning with tongs.