Trim the crusts off the loaf, then cut the bread into slices about 1cm thick. Mix the softened butter with the mustard and spread thinly over half the slices. Make sandwiches by putting the unbuttered slices on top of the buttered ones, then cut each sandwich in half. Place in a lightly greased, 2 litre shallow, ovenproof dish, arranged like a jigsaw so that they fit snugly.
Scatter the leeks and tomatoes over the bread. Whisk the milk with the eggs and season with freshly ground black pepper. Pour over the bread and vegetables, then leave to soak for 20 minutes.
Meanwhile, preheat the oven to 190°C/gas 5. Heat the grill and lightly grill the bacon rashers until just cooked – they will crisp up in the oven. (Alternatively, the bacon can be cooked in the microwave.) Drain on kitchen paper and leave to cool.
When ready to bake, chop the bacon into small pieces and scatter over the bread and vegetables. Sprinkle the grated cheese on top, then bake for 25–30 minutes until puffed and golden. Serve immediately.
For a fishy bake, replace the bacon with 200g smoked haddock fillets, poached in milk, then skinned and flaked. For extra flavour use the poaching milk to mix with the eggs. Scatter the fish over the leeks before soaking with the milk and egg mixture.