Pick through your red lentils to remove any small stones. Rinse your lentils and set aside.
In a large pot heat your olive oil over medium heat and add the diced onion and chopped celery; saute for 2 - 3 minutes. Next add the tumeric, dry mustard and chili flakes. Cook, stirring for an additional 1 - 2 minutes.
To the pot add the remaining ingredients (the cubed carrots and sweet potatoes, the red lentils the vegetable broth and the water) except for the salt.
Cook, stirring occasionally over medium heat for approximately 45 minutes.
Add the salt. Using a handheld immersion blender, or a blender, puree your soup. You can either puree the soup entirely or only half of it if you prefer for your soup to have some texture.