Mussels, if large, cut in half.
Shrimps, if they are large, also cut. If they are small, we will leave it like this. Shrimp and mussels are thoroughly washed and allowed to dry on a napkin.
Meanwhile, prepare the sauce. In our case, this is a simple white instant sauce Bechamel.
In a saucepan or in a frying pan (dry), fry the flour for a couple of minutes, mix. We need the color to darken a little.