Preheat the oven to 200 degrees Celsius. Use a large piece of aluminium foil and make a foil container large enough to place the salmon fillets close to each other and to hold the marinating juices. Place the foil container on a baking sheet and set aside.
Combine the light brown sugar, olive oil, soy sauce, lemon juice and water in a shallow bowl. Coat the salmon fillets both side with the sauce and place it in the foil container, skin-side up. Pour in the sauce and place the lemon slices on top of the salmon fillets if using.
Bake for 15 to 20 minutes in the middle rack until the salmon is cooked through. Transfer the salmon onto a serving plate and drizzle the sauce all over. Serve with baked fries.