Preheat the oven to 350°F (175°C). Gather your baking sheet or dish.

Clean and debeard the fresh mussels under running water, discarding any that are cracked or open. Steam mussels for 5–7 minutes until shells open.

In a mixing bowl, combine melted butter, minced garlic, breadcrumbs, and shredded cheeses. Stir until well combined.

Drain any excess liquid from mussels and lay them shell-side down in the baking dish. Spoon the topping mixture over each mussel.

Bake in the preheated oven for 15–20 minutes until golden-brown and bubbly.

Switch to broil for the last 2–3 minutes for extra crispiness, watching closely to avoid burning.

Remove from oven, let cool slightly, and serve on a large platter.