Prepare the cabbage slaw dressing: add the mustard, vinegar, apple juice/cider and agave to large mixing bowl. While whisking, drizzle in the olive oil until it emulsifies. Add salt and pepper to taste. In the same bowl, add the cabbage, onions, red pepper and apple and toss well, checking for seasonings. Set aside (refrigerate) for at least 30 minutes. On a clean cutting board, set out bread slices to prepare four sandwiches. Spread each slice with mustard, to taste. On each of four of the slices, arrange about 4-6 ounces of corned beef and a large scoop of coleslaw, then sandwich with the remaining bread slices. Cut in half and serve.