With the rack in the middle position, preheat the oven to 425°F (220°C).
In a large pot, cover the potatoes with cold water. Season with salt. Bring to a boil and simmer for 20 minutes or until the potatoes are tender. Set aside in the cooking water.
Meanwhile, in a large skillet over medium-high heat, brown the onion and bell peppers in the oil. Add the water, tomato paste and harissa. Cook for 1 minute. Add the salmon and cook for 4 minutes or until medium-rare. Break the fish into large flakes and season with salt and pepper. Transfer the mixture to a 10-cup (2.5 litre), 8 x 11-inch (20 x 28 cm) baking dish. Layer with the thawed corn and cream corn.
Drain the potatoes and squeeze through a potato ricer directly over the corn in an even layer (see note). Dot the surface with the butter slices.
Bake for 25 minutes or until the potatoes begin to brown. If desired, finish browning under the broiler. Let rest for 10 minutes before serving.
For a richer dish or if a potato ricer is unavailable, mash the potatoes with 1/4 cup (55 g) of butter and 1/2 cup (125 ml) of milk.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.