1. Combine toor dal with 4 cups water and turmeric in medium saucepan. Bring to a boil over medium heat. Reduce heat to medium-low, partially cover, and simmer 45 minutes, or until toor dal is soft. Transfer to blender or food processor, and blend until smooth. Measure, and return to saucepan. Add enough water to make 5 cups.
2. Stir in ginger, serrano chiles, tomatoes, and salt, and bring to a boil over medium heat. Reduce heat to medium-low, and simmer 5 minutes to marry flavors, stirring occasionally.
3. Meanwhile, heat ghee in skillet over medium heat. Add mustard seeds; asafetida powder; red chile; curry leaves, if using; cumin; and pepper. Cover and heat 1 to 2 minutes, or until mustard seeds begin to pop. Pour into soup. Remove soup from heat, and stir in lemon juice. Season with salt and pepper, if desired. Serve garnished with chopped cilantro.