Thinly slice the onion. Cut the scallops in half. Defrost the shrimp, rinse with water, and pat dry.

Heat butter in a frying pan over medium heat. Once melted, add the onion and sauté until softened. Add shrimp and scallops, sauté briefly, then add flour and stir until no longer powdery.

Pour in milk, add salt and pepper, and cook while stirring until the mixture thickens.