To decrease cooking time i like to soak my split peas for a couple of hours (totally optional)
I fill a bowl with split peas and cover it with more then enough water. While the peas are soaking prep the rest of your soup. Chop the onion, slice and wash the leek, chop the carrot, peel the celeriac and cut the carrot.
Now bring the pot to a boil and then turn down the heat to medium/low. leave the soup to simmer for about an hour and keep stirring from time to time to avoid the ingredients to stick to the bottom.
The secret to really nice split pea soup is to stir vigorously. This will help all the ingredients to break down better and create a really nice texture to the soup. If you want it to be silky smooth use a stick blender (i like my pea soup a bit more rustic so i never do this). You can cool the soup and refrigerate or freeze it for later use.