In a large soup pot or Dutch oven, combine the rinsed split peas, diced bacon (or pork belly), and water.

Bring to a boil, then reduce heat to low. Skim off any foam that rises to the surface using a slotted spoon.

Cover and simmer gently for about 45 minutes, stirring occasionally to ensure the peas don't stick to the bottom.

Add the cubed celeriac, sliced leeks, carrots, and potato to the pot.

Continue to simmer uncovered for another 30 to 45 minutes, stirring frequently, until the peas have completely broken down and the soup has thickened to a porridge-like consistency.

Add the sliced smoked sausage and the chopped celery leaves to the pot.

Simmer for a final 10 minutes to heat the sausage through.

Season generously with salt and black pepper before serving hot, ideally with a slice of rye bread.