Rinse mussels under cool water, scrub shells, and discard any cracked or open mussels.

In a large pot, heat olive oil, sauté onions and garlic for 2-3 minutes. Add cleaned mussels and white wine, cover and steam for 5-7 minutes until mussels open.

Transfer mussels to a plate to cool slightly, discard empty shells, and arrange mussel halves on a baking tray.

In a mixing bowl, combine panko, garlic, parsley, and Parmesan. Add lemon zest and juice, olive oil, season with salt, pepper, and optional red pepper flakes.

Fill each mussel shell with the stuffing mixture, pressing gently, and add a pinch of cheese on top if desired.

Preheat oven to 200°C (400°F), bake stuffed mussels for 10-12 minutes until golden brown. Broil for 1-2 minutes for extra crunch.

Remove from oven, let cool slightly, garnish with lemon wedges and parsley, and serve hot.