Preheat oven to 190 C / Gas 5. Grease a 1.2L (2 pt) casserole dish.
Melt butter in a frying pan and add leeks and gammon. Cook and stir for 5 minutes until leeks are soft and gammon is browning.
Cut bread into triangles and layer half the bread in the prepared dish. Scatter half the leek and gammon mixture over the bread and sprinkle with half the grated cheese. Arrange the rest of the bread, followed by the leek mixture and tomatoes on top.
Beat eggs together with the milk and season. Pour evenly into dish and top with the rest of the cheese. Leave to stand for 20 to 30 minutes.
Bake in the preheated oven for 30 to 35 minutes, until puffed up and golden. Serve immediately.
This recipe calls for gammon, however pancetta and streaky bacon would also work well.