Heat 2 tablespoons olive oil over medium-high heat in a large, deep skillet or saucepan. Add the onion, sweet potato, and salt and cook until the onion begins to soften and brown, about 5-7 minutes.
2 tablespoon olive oil, ½ large yellow onion, 1 medium sweet potato, ½ teaspoon kosher salt
Add the garlic, ginger, red curry paste, spices, and sugar to the skillet. Cook, stirring frequently, until the spices begin to brown and become very fragrant, about 2-3 minutes.
2 cloves garlic, ¼ teaspoon ground ginger, 2 tablespoons red curry paste, ½ tablespoon garam masala, 1 teaspoon curry powder, ½ teaspoon turmeric powder, 1 teaspoon granulated sugar, ¼ teaspoon cayenne pepper powder
Pour in the diced tomatoes, scraping the bottom of the pan to remove any fond. Add the red lentils and water into the skillet and stir well to combine. Bring the lentils to a simmer, then reduce the heat to low. Cook covered, stirring occasionally, until the sweet potatoes are tender and the lentils are fully cooked, about 20 minutes.
1 14.5-ounce can diced tomatoes, 1 ½ cup red lentils, 4 cups water
Stir in the coconut milk, spinach, and cilantro. Simmer for an additional 5 minutes or until the spinach has wilted and the lentil mixture has thickened to a desirable consistency. Serve over rice or with warm naan bread.
½ cup coconut milk, 2 cups baby spinach leaves, 2 tablespoons fresh cilantro