Combine water & sugar in a small saucepan over medium heat. Stir until the sugar is completely dissolved & liquid is clear. Whisk in the Italian seasoning & minced garlic. Bring the mixture to a boil for 1 minute, then immediately reduce heat & simmer for 2-3 minutes to help the dried herbs soften & release their oils. Remove from heat, cover & let the sauce steep for at least 20-30 minutes. Pour the sauce through a fine mesh strainer to remove the fine particles of dried herbs.
Salmon/Veggies
Slice salmon fillets into 6 strips. Place them in a small bowl & marinate in some of the herb-garlic sauce for 30 minutes.
Prick potatoes all over with a fork & microwave on HIGH until tender, about 3 min. Cool & cut in half. Set aside.
Preheat oven to 400 F.
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 & mushrooms. Cook until no liquid remains, about 5 min. Cool completely.
Arrange reserved potatoes in 2 au gratin dishes (oval lasagna dishes). Top with the broccoli, mushroom-onion mixture & drizzle with a bit of the herb-garlic sauce. Lay 3 strips of salmon across the top of each dish. Sprinkle with grated cheese.
Bake uncovered until the cheese is golden, the salmon flakes with a fork & the vegetables are tender, about 25 min. Let stand 5 min. before serving.