Step 1

Generously season the flank steak with salt and freshly ground black pepper. Let rest for 30 minutes.

Step 2

Over a preheated medium-high grill, grill the flank steak for about 6 minutes on each side for medium rare. Let the steak rest for 15 minutes. Slice steak into 1/4-inch slices against the grain. Pack the steak for the picnic and keep chilled.

Step 3

Cut the ciabatta rolls in half. Grill lightly until toasted. Let cool and pack for the picnic.

Step 4

Mix together the dijon, vinegar, sugar, mayo, buttermilk, blue cheese, celery salt, and celery seeds in a bowl.

Step 5

Mix together the cabbage, onions, and carrots in a large bowl. Pour the blue cheese dressing on top. Season with salt and pepper. Pack the slaw for the picnic and keep chilled.

Step 6

Layer slices of steak on each roll and top with 1/2 cup of coleslaw. Top with other half of roll.