In a large pot, add the fresh mussels along with a small amount of water. Cover the pot and cook over medium-low heat for about 5-7 minutes, or until the mussels' shells open. Remove the pot from heat and let the mussels cool. Carefully take off one half of the shells, keeping the mussel meat intact.

In a saucepan, melt 100 grams of butter over medium heat. Add the minced garlic and sauté until fragrant, about 1-2 minutes. Stir in breadcrumbs and toast for an additional minute. Gradually pour in milk, stirring continuously until the sauce thickens. Season with salt before removing from heat.

Preheat your oven to 355°F (180°C). Line a baking tray with parchment paper. Arrange the mussel shells upright and spoon garlic-bechamel sauce over the mussels. Sprinkle grated quick-melt cheese on top.

Transfer the mussels to the oven and bake for 10-15 minutes until the cheese is melted and bubbly, achieving a light golden brown color.