- Step 1
Melt butter in a saucepan over medium heat. Add flour. Cook, stirring, for 1 minute or until bubbling. Remove from heat. Gradually add milk, stirring to prevent lumps forming, until combined. Return pan to medium heat. Cook, stirring, for 3 minutes or until sauce boils and thickens. Add cheese and mustard. Stir until cheese has melted. Remove from heat. Stir in egg yolk. Season with salt and pepper.
- Step 2
Preheat grill on medium-high. Line a baking tray with foil. Place bread on prepared tray. Grill bread for 1 to 2 minutes or until light golden. Remove from grill. Turn bread over. Top with ham and tomato. Spoon over cheese mixture. Grill for 3 to 4 minutes or until bubbling and golden. Serve.