Roughly chop the bacon, onions, carrots, leeks and celery, and then set half of the bacon aside.
Heat a tablespoon of the oil and the butter in a large heavy saucepan then add the bacon and fry for 2 minutes.
Tip in the vegetables and cook, stirring occasionally, for 10 minutes.
Add the split peas and stock then add the lid.
Bring the mixture to a boil then simmer or an hour until the peas are tender.
When the soup has finished cooking let the mixture cool for a few minutes.
Use an immersion or stick blender to blend the mixture.
Slice off the crusts from sliced bread and cut into cubes about 1 cm or half an inch.
Heat the remaining butter and oil in a frying pan over a medium heat and add the cubes of bread.
Fry for a few minutes, turning regularly, until they are golden and crisp.
Tip them out onto kitchen paper to drain.
Use the same pan to dry fry the bacon, and then cut it into pieces.
Serve the soup with the croutons and bacon sprinkled over the top.