Dutch Erwtensoep (Pea Soup)

Rinse the peas, remove any bad ones and soak in 2 l of water, with salt, for 12 hours. Bring the peas and the salt water to a boil and simmer for about 2 hours until tender and cooked. Add the bacon and pork trotter after 1 hour.
After the 2 hours, remove the meat and pour out the cooking liquid (set aside). Mash the peas to your preferred consistency. Put the cooking liquid and meat back into the pot. Also add the potatoes, celeriac, leeks and celery. Season with salt and pepper, and add some more water if the soup is too thick. While stirring, bring the soup back to a simmer and cook until the vegetables are tender.
Meanwhile heat the smoked sausage (follow instructions on packaging) and slice. Take the bacon and pork trotter out of the pot. These can be served on the side, or sliced and put back into the soup.
If preparing in advance, refrigerate everything and prepare the smoked sausage when serving. Serve in big bowls with slices of rye bread (topped with the bacon or the sausage, and some mustard).