Combine the rinsed toor dal, 3 cups of water, a pinch of salt, and 1/2 teaspoon of turmeric in a pot. Bring to a boil, then reduce heat to medium-low, cover, and simmer for 15-20 minutes until the lentils are soft, mushy, and break down easily.
Heat 2 tablespoons of ghee in a separate pan over medium heat. Add the cumin and mustard seeds, letting them sizzle and pop for about 10-15 seconds to release their natural oils.
Add the finely chopped onions and slit green chilies. Sauté, stirring occasionally, until the onions turn a rich golden brown color (about 6-8 minutes).
Stir in the ginger-garlic paste and cook for 1 minute until the harsh raw aroma dissipates.
Add the chopped tomatoes, red chili powder, and a pinch of salt. Cook down until the tomatoes turn soft and jammy, and you see tiny droplets of ghee separating at the edges of the pan.
Pour the boiled yellow dal into the sautéed masala mixture. Stir well to integrate the flavors. If the dal is too thick, add a splash of warm water.
Let the dal simmer gently on low heat for 5 minutes. Turn off the heat, stir in the garam masala, crushed kasuri methi, chopped cilantro, and fresh lemon juice. Cover for 2 minutes before serving.