Pour oil or ghee into a dutch oven and place over medium-high heat.
Add sweet potatoes and sauté for 2 to 3 minutes. Stir in onions, garlic, ginger, and Serrano chile and continue to sauté for 4 to 5 minutes. Season with salt and pepper.
Stir in red lentils, cumin, coriander, turmeric, ginger, and fenugreek leaves and continue to cook for 4 to 5 minutes, until mixture becomes fragrant and slightly smoky in scent.
Add diced tomatoes and stock and bring mixture to a boil. Once mixture is boiling, cover and reduce heat to medium-low and simmer for 30 to 35 minutes or until lentils have softened and mixture has turned into a stew-like consistency. Season with salt and pepper and stir in cilantro.
Top with more cilantro and serve with a side of grilled naan or steamed basmati rice.
Notes: If you'd like your masoor dal more (traditionally) soupy, add an additional 2 to 3 cups of stock to your pot. To Freeze and reheat: Completely cool dal before transferring into airtight containers. When ready to use, transfer the frozen dal into a pot along with 1/2 to 2 cups water or stock. Melt over medium-high heat and boil until completely heated through. Season as needed with salt and pepper. Serve.