Pick over the peas, wash, cover with cold water, and soak overnight.
Simmer the peas along with the salt pork, vegetables, and salt until tender, using the same water in which the peas were soaked. Add the garlic and press these ingredients through a sieve.
If the soup is thick, add some boiling water to get the desired consistency.
Brown the flour very lightly in the fat; add the grated onion and cook for a few minutes. Pour some soup liquid into it, stir until smooth and add to the soup. Bring to a boil. Season to taste with salt and pepper.
Place a tablespoon of the finely diced dill pickle into each serving of soup. Serve with croutons.