In a large saucepan, heat the oil over medium heat. Add the onion and garlic and sauté for 5 to 6 minutes, until translucent.
Stir in the ginger, 1 tablespoon of the curry powder, coriander, and cumin and sauté for 2 minutes more, until fragrant.
Add the broth and red lentils and stir to combine. Bring the mixture to a low boil, then reduce the heat and simmer for 5 minutes more.
Stir in the cauliflower and sweet potato. Cover and reduce the heat to medium-low. Simmer for 20 to 25 minutes, until the cauliflower and sweet potato are tender. Season with the salt and pepper, and add more curry powder, if desired. Stir in the spinach and cook until wilted.