- To make the bechamel sauce, put the milk in a saucepan and heat to just below boiling point, remove from heat.
- Put the butter in a medium saucepan, and cook over low heat until melted. Add the flour and cook, stirring, for 2 minutes. Gradually add the milk, beating well with a wooden spoon (or silicon spatula) until very thick between additions. Cook, stirring over low heat for 10-15 minutes until very thick. Stir in Dijon mustard. Season to taste with salt, refrigerate in a shallow bowl until chilled completely.
- Place the béchamel sauce, ham, gruyere cheese and onion in a bowl and scoop a large tablespoon of mixture into lightly floured hands and shape into about 10 croquettes (short, sausage shapes).
- For the crumbing, lightly beat the eggs in a shallow bowl. Place the flour and breadcrumbs in separate small bowls, then roll each croquette in the flour, then the egg mixture and breadcrumbs, Refrigerate for about 30 minutes.
- Pour enough oil into a large frying pan to come 5mm up the side of pan. Heat over medium-high heat. Cook croquettes, turning, for 4 to 5 minutes or until golden. Using a slotted spoon, transfer to paper towel to drain.
- Serve croquettes with salad.