Prick potatoes all over with a fork and microwave on HIGH until tender, about 3 min. Cool and slice thinly. Set aside.
In a large non-stick skillet, heat oil over medium-high heat. Cook the onion until golden, about 5 min. Stir in garlic, salt, pepper and mushrooms. Cook until no liquid remains, about 5 min. Cool completely.
Arrange reserved sliced potatoes in a greased 5 x 8-in. (12 x 20 cm) freezer-proof and ovensafe baking dish. Top with the broccoli, mushroom-onion mixture and the salmon. Sprinkle with cheese, cover with foil and freeze.
Bake uncovered in a preheated 400°F (200°C) oven until the cheese is golden, the salmon flakes with a fork and the vegetables are tender, about 35 min. from frozen or 25 min. if baking from fresh. Let stand 5 min. before serving.
This layered recipe can easily be doubled. To change up colour and texture, substitute 1/2 cup (125 mL) of cauliflower for the broccoli. NOTE: Total Time: 55 min. from frozen, 45 min. from fresh.