Split Pea and Potato soup

I found this in an old Reader's Digest Healthy cookbook that I got at half-price books. I thought it was really good especially with my home made croutons. I didn't have any low-sodium broth, the regular stuff had to do.

1 Tbsp margarine
1 med onion, chopped
2 cups low-sodium beef broth
2 cups water
1/2 c dried split green peas, rinsed and sorted
2 medium potatoes, peeled and quartered (I diced them)
1/4 tsp ground black pepper

In a large heavy saucepan, melt the margarine over medium heat. Add the onion and cook uncovered until soft - about 5 minutes. Stir in the beef broth and water and bring to a boil - about 4 minutes. Add the peas and potatoes, adjust the heat so the mixture bubbles gently, cover and cook for 30 minutes or until the peas and potatoes are tender. Remove from heat and cool for 10 minutes.

*In an electric blender or food processor, puree the soup in 5 batches, whirling each batch about 15 seconds. Return to the pan, set over low heat and bring to serving temperature, stirring often. Add the pepper.

*I have an immersion blender so I just pureed the soup in the pan, no need to cool or reheat.