For the pesto, place all the ingredients except the oil in a food processor and blend. Gradually add the oil until you have a thick paste, and set aside.
For the sandwich, place the red and yellow peppers directly on a gas flame and keep turning until black all over. Remove from the hob and place in a bowl, cover with cling film, and set aside. Once cool enough to handle, peel off the black skin and cut the flesh into quarters, removing the seeds.
Heat a griddle pan and drizzle the sliced courgettes with olive oil. Place on the griddle until charred on both sides.
Slice off the top of the loaf of bread. Hollow out the loaf by scooping out the soft bread, leaving 3cm/1¼in of bread around the edge.
Fill the hollow loaf with layers of the peppers, tomatoes, courgettes, red onion, cheese and pesto, seasoning between each layer.
Place the lid of the loaf back on and push down, you can wrap it in cling film and leave overnight or slice straight away.