Preheat oven to 400°F.
Heat the olive oil over medium-high heat in a large skillet. Stir in 2 tablespoons of the butter until melted. Add onion, bell pepper, and celery, and cook until soft, about 5 to 7 minutes. Add the scallions and garlic and cook for another minute.
Add oysters and mushrooms and bring to a simmer, stirring often, 5 minutes. Set aside.
Melt 2 tablespoons of the butter in a medium saucepan over medium heat. Whisk in flour, and cook, whisking constantly, until smooth, 1 minute.
Stir in half and half, lemon juice, Worcestershire sauce, hot sauce, salt, and pepper, whisking constantly, until very thick and beginning to bubble, about 2 to 5 minutes. Stir in Parmesan cheese until melted, about 1 more minute.
Using a fine-mesh sieve or colander, strain the oyster mixture, and discard the liquid.
Add oyster mixture to cheese sauce and stir until fully incorporated.
Spread mixture in an 11x7-inch baking dish.
Melt the remaining 2 tablespoons of butter, then toss breadcrumbs with melted butter and a pinch of salt. Sprinkle the breadcrumbs over the oyster mixture.
Bake for 15 minutes, or until bubbly. Remove the casserole from the oven and move the rack to the 2nd level under the broiler. Place the casserole under the broiler for 1 to 2 minutes, until the breadcrumb topping is slightly browned. Watch closely!!! Don't let burn, as broilers vary! Remove from oven and serve at once.