Set the oven to 375°F (190°C). Grease a medium casserole dish with butter or nonstick spray.
In a bowl, combine melted butter, garlic, breadcrumbs, onion or shallot, parsley, lemon juice, salt, pepper, and Parmesan (if using). The mix should be moist but not wet—add more butter if it feels dry.
Flatten the butterflied shrimp and press 1–2 teaspoons of stuffing into each. Gently fold the shrimp around the filling.
Place shrimp in the dish, stuffing side up. Keep them close together to hold the filling in place.
Sprinkle remaining stuffing over the shrimp. Drizzle with extra melted butter or olive oil.
Bake for 18–20 minutes, until shrimp are pink and stuffing is golden. The top should be crisp and fragrant.
Top with fresh parsley and lemon wedges. Serve hot and enjoy.