Begin by thoroughly cleaning your mussels; remove the beards and scrub the shells under cold running water using a brush. Soak the mussels in a bowl of fresh water for about 20 minutes.

In a large pot, bring a small amount of water to a gentle simmer over medium-low heat. Add the cleaned mussels and cover the pot, allowing them to steam for about 5-7 minutes.

In a saucepan over medium heat, melt the butter. Add minced garlic and sauté until fragrant. Stir in breadcrumbs and cook for a minute. Add milk gradually, whisking until thickened. Season with salt.

Arrange the cleaned mussel halves on a baking tray. Spoon a generous dollop of the bechamel sauce into each shell and top with grated cheese.

Preheat your oven to 355°F (180°C). Bake the stuffed mussels for 10-15 minutes until cheese is melted and golden brown.