Sort and rinse the red lentils and place in bowl and cover with water. Set aside.
Put the water on the stove in a large pot or Dutch oven over medium heat, then add the marrow bone. Add the salt and bring to a boil.
Once the water starts to boil, remove the scum and reduce the heat so that the bone broth is simmering. Rinse the barley, and add to the pot.
Meanwhile, chop the onion, celery, carrots and rutabaga into similar sized pieces. In a large sauté pan over medium heat, add the olive oil and and then the chopped vegetables.
Sauté, stirring often, adding a sprinkle of salt. Do not brown the vegetables. After about 8-10 minutes, ladle some of the broth to cover the veg, without any barley, and continue to cook. Cover and simmer until the vegetables are cooked through.
While the vegetables are cooking, drain the red lentils and add to the pot of broth. Stir well and continue to simmer.
When the vegetables are ready, place in a blender or container and blend until smooth. Add the puréed vegetables to the pot of marrow bone soup. Stir and continue to simmer. The soup should simmer for about one and a half hours in total.
Taste for salt, adding as needed, along with freshly grated black pepper, fresh parsley. Remove from heat when ready. See notes for the bone marrow.
Serve hot, adding extra parsley into individual bowls, if desired.