In a large soup pot, or dutch oven, heat oil over medium heat. Add cumin seeds. Heat about 1 to 2 minutes. Do not let cumin seeds burn.
Add onion, garlic, and ginger. Cook over medium-low heat with the lid on for 2 to 3 minutes until the 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. Stir in coconut milk, if using.
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 working in batches until you reach desired consistency.