1. Bring a saucepan of lightly salted water to the boil, add the sprouts; cook until crisp-tender.
2. Melt butter in a saucepan over moderate heat. Stir in flour. Cook for 1 minute, then gradually add milk and stock, stirring until mixture boils and thickens. Stir in mustard and mayonnaise.
3. Drain sprouts, tip into a heated bowl and pour over sauce.