Clean the mussels thoroughly and remove any beards. Place them in a pot with a little water, cover, and cook over low heat for 5-7 minutes until the shells open. Remove one half of each shell and set aside.

In a medium saucepan, melt 2 tablespoons of butter over medium heat. Sauté minced garlic for 1 minute, then stir in breadcrumbs for another minute. Gradually add 1 cup of milk, whisking continuously until the sauce thickens, about 5 minutes. Season with salt and let cool slightly.

Preheat the oven to 355°F (180°C). Line a baking tray with parchment paper, arrange cleaned mussel shells on the tray, and fill each shell with the garlic bechamel sauce. Top with grated cheese.

Bake the mussels in the preheated oven for 10-15 minutes until cheese is melted and golden brown. Remove from oven and let cool slightly before serving.