Toast the bread lightly so it holds its shape, then lay the slices on a grill pan.
Gently melt the butter in a saucepan, stir in the flour and cook for 1 minute, then add the milk a little at a time, stirring until you have a smooth, thick sauce.
Take off the heat and stir in the cheese, mustard, Worcestershire sauce and plenty of black pepper until you have a thick, glossy paste.
Spoon the cheese mixture generously over each slice of toast, spreading it right to the edges.
Grill for 4-5 minutes until bubbling, blistered and golden in patches. Serve straight away with the sliced tomato.