Place the rinsed lentils, cubed sweet potatoes, diced onion, minced garlic, and grated ginger into the slow cooker.
Add the curry powder, cumin, vegetable broth, and diced tomatoes. Stir everything together.
Season with salt and black pepper, then cover the slow cooker.
Cook on low for 6 to 8 hours or on high for 3 to 4 hours until the lentils and sweet potatoes are tender.
Stir in the fresh spinach during the final 10 minutes of cooking so it wilts evenly.
Ladle the stew into bowls and top each serving with fresh cilantro.