Put the dal and water into a 4-quart pot and bring to a boil over high heat. Skim the scum and then add the tomato. Lower the heat to simmer. Cover and cook until the dal is soft and broken up, 40 to 60 minutes. Remove and discard the tomato skin pieces, and then gently whisk to further blend the dal and tomato. The dal will break up a bit.

You can make the dal a day in advance up to this point. Let it cool completely and refrigerate. Return it to room temperature before moving forward. Add a splash of water when bringing it up to a simmer as it’s bound to thicken a lot from sitting overnight.

Bring the dal to a simmer and have a lid ready before proceeding. Heat the oil in a medium skillet over medium-high heat until nearly smoking. Add the cumin and mustard seeds and partially cover to shield yourself from the sizzling drama. Decrease the heat to medium. After the seeds stop sputtering, uncover.

Add the garlic and onion (or the chiles and curry leaves). If you used onion, cook, stirring for a good 5 minutes, until the onion has turned a dark brown. If you opted for the chiles and curry leaves, you only need to cook them with the garlic for about 1minute, or until the chiles and leaves are fragrant and slightly blistered.

Regardless, turn off the heat and stir in turmeric, coriander, and cayenne. Add to the simmering dal. Stir in the salt and butter. Remove from the heat and set aside for 5 minutes to develop the flavor. Taste and add extra salt, as needed. Transfer to a shallow bowl, garnish with cilantro, and enjoy with rice, naan, chapati, or warm whole-wheat tortillas.