Melt butter and add bacon, onion, carrot and celery. Cook for 5-10 minutes until the vegetables have begun to soften.
Add the peas and the stock. Bring to a boil, then cover and simmer 1 hour, until the peas are cooked. Place in a blender to liquify (use caution with hot soup). Return to the pot and add the yogurt; gently reheat without boiling.
Serve garnished with crispy, crumbled bacon and croutons if you like.