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, mix together the mayonnaise and harissa. Set aside.
In another bowl, toss the breadcrumbs with the butter. Set aside.
Lightly oil the asparagus.
Remove the baking sheet from the oven. Gently add the asparagus to the potatoes. Place the fish among the vegetables, making sure the undersides are in contact with the sheet. Season lightly with salt. Brush the fish with 1 tbsp (15 ml) of the spicy mayonnaise, then cover with the breadcrumbs, pressing lightly so they adhere.
Bake for 8 to 10 minutes or until the fish is fully cooked and the breadcrumbs are light golden brown. Finish under the broiler, as needed.
Serve the fish and vegetables with the remaining mayonnaise.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.