- 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.