Rinse the red lentils in a fine-mesh sieve under cold water until the water runs clear, about 1 minute.

Add the lentils to a 4-quart / 3.8 L saucepan with the water or broth, turmeric, minced garlic, 2 tsp grated ginger, diced onion, ground cumin, and 1 tsp salt. Bring to a boil over high heat.

Reduce heat to low, cover partially, and simmer for 20 to 25 minutes, stirring every few minutes, until the lentils are soft and have broken down into a thick, porridge-like consistency.

While the lentils simmer, heat the coconut oil in a 20 cm / 8-inch skillet over medium heat.

Add the cumin seeds and mustard seeds. Cook for 30 seconds until they sputter and smell toasty.

Add the sliced garlic, remaining 1 tsp ginger, and chili. Cook 45 seconds to 1 minute until fragrant, watching closely since the garlic can burn and turn bitter within a minute.

Stir in the diced tomatoes and cook for 5 to 6 minutes, stirring occasionally, until they break down and the oil separates at the edges of the pan.

Scrape the tempering into the pot of lentils and stir to combine. Simmer uncovered for 5 more minutes so the flavors meld.

Stir in the coconut milk if using, and season with salt to taste. If the dahl looks too thick, thin it with a splash of hot water since it keeps thickening as it sits.

Ladle into bowls, top with chopped cilantro, and serve with lime wedges on the side.