Preheat your oven to 190°C (375°F). Lightly butter a 20cm x 20cm (8x8 inch) baking dish or a similar-sized shallow casserole dish. This prevents sticking and ensures an even bake.

Prepare the oysters: Gently drain the shucked oysters, being careful to collect all the liquor in a separate bowl. Inspect the oysters for any shell fragments and remove them. If the oysters are very large, you can cut them in half, but keep them whole if they are bite-sized. Set aside.

Make the creamy sauce base: Melt 50g (1/4 cup) of butter in a medium saucepan over medium heat. Add the finely chopped yellow onion (150g or 1 cup) and celery (75g or 1/2 cup). Sauté for about 5-7 minutes, until the vegetables are softened and translucent, but not browned. Stirring occasionally prevents burning.

Create the roux: Sprinkle the 30g (1/4 cup) of all-purpose flour over the softened vegetables and butter. Stir continuously for 1-2 minutes to cook out the raw flour taste, forming a paste. This is your roux, which will thicken the sauce.

Gradually whisk in the liquids: Slowly pour in the 250ml (1 cup) of heavy cream, 125ml (1/2 cup) of milk, and the reserved 125ml (1/2 cup) of oyster liquor. Whisk constantly to prevent lumps from forming. Bring the mixture to a gentle simmer.

Simmer and season the sauce: Continue to cook the sauce over medium-low heat, stirring frequently, for about 5-8 minutes, or until it has thickened enough to coat the back of a spoon. Do not let it boil vigorously. Stir in 1 tablespoon (15ml) of Worcestershire sauce, 2 tablespoons (30ml) of dry sherry or white wine (if using), 1/2 teaspoon of freshly ground black pepper, and 1/4 teaspoon of salt. Taste and adjust seasoning if necessary. Remember that oysters are naturally salty, so be cautious with added salt.

Add oysters and parsley: Gently fold the prepared oysters and 2 tablespoons (15g) of chopped fresh parsley into the thickened sauce. Stir just enough to combine. Be careful not to overmix or break up the oysters.

Assemble the casserole: Pour the oyster and sauce mixture evenly into the prepared baking dish. Ensure the oysters are mostly covered by the sauce.

Prepare the breadcrumb topping: In a small bowl, combine 75g (1 cup) of breadcrumbs with 30g (2 tablespoons) of melted butter. Toss until the breadcrumbs are evenly coated. This ensures a crispy, golden topping.

Top and bake: Sprinkle the buttered breadcrumbs evenly over the oyster mixture in the baking dish. Place the dish on a baking sheet to catch any potential drips.

Bake the scalloped oysters for 20-25 minutes, or until the topping is golden brown and crisp, and the sauce is bubbly around the edges. The oysters should be cooked through but still tender. Avoid overbaking, which can make the oysters tough.

Rest and serve: Remove the scalloped oysters from the oven and let them rest for 5-10 minutes before serving. This allows the sauce to set slightly. Garnish with a little extra fresh parsley if desired. Serve hot.