Prick potatoes all over with a fork and microwave on high until tender, about 3 minutes. Cool and slice thinly. Set aside.
In a large nonstick skillet, heat oil over medium-high heat. Cook the onion until golden, about 5 minutes. Stir in garlic, salt, pepper and mushrooms. Cook until no liquid remains, abour 5 minutes. Cool completely.
Arrange reserved sliced potatoes in a greased 5x8-inch freezer proof and oven safe baking dish. Top with the broccoli, mushrooms-onion mixture and the salmon. Sprinkle with cheese, and cover with foil. Freeze.
Bake uncovered in a preheated 400F oven until the cheese is golden, the salmon flakes with a fork and the vegetables are tender, about 35 minutes from frozen, or 25 minutes if baking from fresh. Let stand 5 minutes before serving.