Preheat your oven to 375°F (190°C).

In a skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until it becomes translucent.

Add the minced garlic and sliced mushrooms to the skillet. Cook until the mushrooms are tender and have released their moisture, about 5-7 minutes.

Stir in the heavy cream and season with salt and pepper. Let it simmer for 3-4 minutes until slightly thickened. Remove from heat.

In a baking dish, place the fish fillets. Squeeze lemon juice over the top and season with salt and pepper.

Pour the mushroom and cream mixture over the fish fillets, ensuring they are well covered.

In a bowl, combine the grated Gruyère cheese and breadcrumbs. Sprinkle this mixture evenly over the mushroom gratin.

Bake in the preheated oven for 25-30 minutes, or until the fish is cooked through and the top is golden brown.

Remove from the oven and let it sit for a few minutes before serving. Garnish with fresh parsley.