Place the potatoes in a pot and cover with cold water. Season with salt. Bring to a boil and simmer for 20 minutes or until tender. Drain. Return the potatoes to the pot. With a potato masher, mash the potatoes with the milk and butter until smooth. Add the cheese and egg. Mix vigorously with a wooden spoon. Season with salt and pepper. Cover and set aside.
With the rack in the middle position, preheat the oven to 375°F (190°C).
Meanwhile, in a food processor, finely chop the mushrooms, onion and garlic.
In another large pot over high heat, brown the mushroom mixture in the oil. Add the buckwheat and cook for 2 minutes. Add the water and soy sauce. Simmer for 15 to 20 minutes, stirring occasionally, or until the liquid has completely evaporated. Season with pepper.
Spread the buckwheat mixture over the bottom of an 11 x 8-inch (28 x 20 cm) baking dish. Cover with the cream-style corn and corn kernels. Top with the mashed potatoes. Using a fork, make a decorative pattern in the mashed potatoes. Sprinkle with paprika.
Bake for 25 minutes. Finish cooking under the broiler for 3 minutes or until nicely browned. Let rest for 10 minutes.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.