In a large soup pot, or dutch oven, heat oil over medium heat. Add mustard seeds and cumin seeds. Heat until mustard seeds begin to pop, about 1-2 minutes. Do not let cumin seeds burn.

Add onion, garlic, and ginger. Cook over medium-low heat with lid on for 2-3 minutes until onion begins to soften. Add potatoes and red lentils and stir to coat. Add broth and bring to a gentle simmer. Cover pot and cook over medium low heat for 15-20 minutes until potatoes and lentils are very soft and mashable.

Add salt, black pepper and turmeric to taste. If using coconut milk stir in to incorporate now. The soup will be chunky at this point. Eat as is, or use an immersion blender to blend until creamy. You can also use a regular blender, but you will have to blend in batches once the soup has cooled a bit. Keep warm on low heat.