Prepare the seafood by drying your mixed seafood, cutting the white fish into bite-sized chunks, and seasoning generously with salt and pepper.

Preheat your oven to 200°C (400°F) and heat olive oil in a large skillet over medium heat. Add onion and sauté for about 5 minutes until soft, then stir in minced garlic and cook for 1-2 minutes.

Add butter to the skillet, melt it, then whisk in all-purpose flour to create a roux. Gradually pour in milk and heavy cream while whisking for about 3-4 minutes until thickened.

Remove from heat and gently fold the seasoned seafood into the creamy sauce, adjusting seasoning as needed.

Transfer the mixture into a greased baking dish, sprinkle cheese and breadcrumbs on top evenly, optionally mixing breadcrumbs with melted butter for extra crunch.

Bake in the preheated oven for 20-25 minutes until the top is golden brown and edges are bubbling.

Let the gratin rest for 5 minutes before serving and garnish with freshly chopped parsley.