Heat oil in a medium pot set over medium heat. Add onion and sauté for 5 minutes. Add garlic and ginger and sauté for 2 minutes. Add cumin and turmeric and sauté for 1 minute, stirring constantly.
Add sweet potatoes, lentils, and enough water to just cover ingredients in pot.
Simmer until lentils and sweet potato are tender, approximately 20 to 25 minutes.
Add spinach and simmer for a few minutes, or until spinach is soft and wilted. Season with salt and pepper and serve.