Heat a frying pan and cook caster sugar for 2 minutes until melted and golden-brown.
Add tomatoes, vinegar, chilli, apple, and thyme, then cook for 3 minutes and season.
Preheat the grill to a high setting.
Heat cheese and milk in a saucepan until the cheese has melted.
Stir in the flour and breadcrumbs and cook for 3 minutes.
Remove the pan from the heat and allow the mixture to cool.
Whisk in the Worcestershire sauce, egg, egg yolk, mustard, and seasonings.
Toast one side of the bread, spread cheese mixture on the other, and grill for 2 minutes.