Preheat your oven to 400°F (200°C). Arrange the rack in the lower third of the oven. Shuck each oyster, keeping their shells for later use.
In a pan over medium heat on stove top or microwave-safe dish if using microwave, melt butter until it's golden brown and has nutty undertones (about 5 minutes). Stir in chervil, parsley, button mushrooms, cheese blend, breadcrumb mixture, salt, pepper. Remove from heat.
Pour the oyster filling into their shell halves on a baking sheet or tray lined with aluminum foil to catch any spills (this also makes clean-up easier). Sprinkle extra Parmesan cheese over each filled half and place under broiler for about 2 minutes, until the top is lightly browned.
Place them on their sides in an ovenproof dish or back into baking sheet if using one. Broil again briefly to brown more (keep a close watch). Serve immediately.