Preheat the oven to 425 degrees . Line a baking sheet with parchment paper. In a large bowl, stir together the mayonnaise, yogurt and lemon juice; season with salt and pepper. Transfer 1/4 cup of the dressing to a small bowl.
Using a food processor fitted with the shredding disk, shred the broccoli stalks. Add to the dressing in the large bowl, then add the coleslaw mix and toss. Season with salt and pepper.
Wipe out the food processor and fit with the blade; add the potato chips and finely chop. Add the breadcrumbs and process until finely ground. Transfer to a medium bowl. Coat the fish fillets with the egg, then with the potato chip-breadcrumb mixture. Arrange on the prepared baking sheet and bake until firm, about 12 minutes.
Spread the reserved dressing on the insides of the rolls. Cut the fish into 6 portions and place on the roll bottoms; top with some slaw. Serve the sandwiches with the extra chips.