Fry the pancetta in a large saucepan with the butter until golden and crisp.
Remove two thirds onto kitchen paper and keep for later. Add the onion to the pan and fry for 5 minutes.
Add the celeriac and the potato chunks into to the pan with the stock. Simmer for 15 minutes or until the veg are tender.
Meanwhile, preheat the grill. Place the bread cut into 1 inch pieces on baking sheet and brush with a little olive oil. Sprinkle with half the poppy seeds, then toast under grill until lightly browned.
Whizz the soup with a hand blender until smooth.
Taste and season, then serve topped with dollops of crème fraîche, the croutons, rest of the pancetta, extra-virgin olive oil, parmesan cheese and fresh watercress, if you like.