Slice off the top third of the loaf. Scoop out the inside of the loaf – and some of the bread from the lid – leaving a 2cm edge of bread all around.
Arrange the tomatoes inside, then top with the mozzarella, spinach, artichokes and prosciutto. The layers should come to just above the top.
Put the lid back on the loaf and wrap with foil or clingfilm. Place an upside-down plate on top, with a weight (such as a tin of beans) on top of that. Leave for a few hours or overnight in the fridge to press the layers together.