Wash the fish fillets, pat dry with paper towels, place in a greased baking dish, and drizzle with lemon juice.
Preheat the oven to 180°C (350°F) with convection, or 200°C (400°F) with conventional heat.
Place the baking dish in the oven and bake for 15 minutes.
While the fish bakes, prepare the sauce: in a saucepan, mix the cold whipped cream with the flour, add the diced cheese, and heat over medium heat, stirring constantly until the mixture comes to a gentle boil and the cheese melts completely.
Season the sauce with salt, pepper, and lemon juice to taste, stir in the drained mushrooms and parsley, then taste and adjust seasoning if needed.
Remove the fish from the oven, pour the sauce over it, return to the oven, and bake for another 15 minutes until the sauce is lightly browned and bubbly on top.