Steam vegetables until tender (15 minutes) and then arrange vegetables and apples around the edge of a large platter.
Meanwhile prepare fondue; toss cheese and flour into a large bowl.
Bring the stout, apple juice and mustard to a simmer in a saucepan over medium heat. Gradually add cheese mixture to pan; stirring constantly until cheese is melted and smooth (you may need to thin with some additional stout). Season with salt and pepper, if desired.
Transfer fondue to bowl or fondue pot and place in center of platter with vegetables.