To a dutch oven or soup pot over medium heat, add the oil. Sauté the onions in the oil until softened, about two minutes. Add the garlic, then stir in the red curry paste, continuing to cook another one minute. Add the sweet potatoes, can of diced tomatoes, and lentils to the pan, stirring to combine. Season with salt. Pour the chicken into the pan and bring to a boil. Reduce the heat to medium-low and simmer 20 minutes or until the lentils and sweet potatoes are tender. Stir in the coconut milk and lime juice. Season with salt and pepper to taste. Serve with a sprinkle of fresh cilantro over each bowl.