Start by cleaning the celeriac and squash. In a large saucepan melt some butter and add the chopped onion. Let it fry until the onion is soft but not brown. Ad the pumpkin and squash and fry for a minute or two before adding the vegetable stock. Don’t add it all at the same time. I added about 500 ml to begin with but once the veggies get cooked and you have an idea of the thickness of the soup you can add more stock if needed. I like my pumpkin soup to be fairly substantial but if you want it thinner you can add more fluids.
Once the vegetables are soft puree them either in a blender or with a stickmixer (is that even a word?? I sort of feel it has a different name in English) and add seasoning where you want.
Cut the bread into small cubes and fry them in oil in a frying pan. Once golden drain on kitchen paper. Put the soup in a bowl and add croutons and garden cress on top.
If you want to give the croutons some extra funk you can add grated parmesan. I pour some olive oil over the bread and grate lots of parmesan over it. Mix it all and then continue to fry them in the pan.