In a large pot, heat the oil over medium-high heat. Add the sweet potato and saute for 6 minutes or until beginning to soften. Add the onion and saute for an additional 4 minutes or until translucent. Season with salt and pepper.
Add the garlic, ginger, curry powder, cumin, coriander, turmeric, and cinnamon and saute for 2 minutes or until fragrant. Add the tomatoes, lentils, and water and bring to a boil. Reduce the heat to low and simmer for 10 minutes or until thickened. Stir in the spinach, and cook over low for 1-2 minutes or until wilted. Season to taste with salt and pepper and a squeeze of lemon.
Serve with a dollop of yogurt (if desired) and some cilantro leaves.
*I used tiny orange lentils (known as “petite crimson lentils”) which cook much quicker than regular lentils. You can any type of lentil, but you’ll need to increase the cooking time from 10 minutes to about 30 minutes.