1 1/2 lb (675 g) hake or haddock fillets, cut into 4 pieces
With the rack in the middle position, preheat the oven to 450°F (230°C). Line a baking sheet with parchment paper.
Toss the potatoes with the oil and season with salt and pepper. Place on the baking sheet cut side down. Bake for 20 minutes.
In a small bowl, combine the mayonnaise and harissa. Set aside.
In another bowl, combine the breadcrumbs and butter. Set aside.
Lightly oil the asparagus.
Remove the baking sheet from the oven. Gently toss the asparagus with the potatoes. Nestle the fish among the vegetables, making sure the undersides of the fillets are in contact with the baking sheet. Season lightly with salt. Brush the fish with 1 tbsp (15 ml) of the spicy mayonnaise, then cover with the breadcrumbs, pressing lightly to adhere.
Bake for 8 to 10 minutes or until the fish is fully cooked through and the breadcrumbs are lightly golden. Finish under the broiler, as needed.
Serve the fish and vegetables with the remaining mayonnaise.