Make the dal. Rinse the lentils in a fine-mesh sieve until the water runs mostly clear.
Add lentils to a medium saucepan and cover with the water. Bring to a boil, and use a spoon to skim off foam as it comes to a boil. Reduce the heat to a simmer, stir in the turmeric, and partially cover the pan.Simmer for 20 to 30 minutes (split lentils should be done around 20 minutes), or until lentils are soft and tender and mostly broken down.Note: While the lentils cook, prep the ingredients for the Spinach Tadka. Now whisk vigorously until the lentils are somewhat pureed, about 1 minute. (If they aren’t easily pureed, cook for a bit longer.) Cook for a few more minutes, whisking occasionally, until the lentils are completely soft and mashed and the dal has thickened.
Stir in the salt and lemon juice. Taste, adding more salt or lemon as desired. Keep the dal warm over the lowest heat while you cook the Spinach Tadka.
Make the Spinach Tadka. Heat a medium or large frying pan over medium-high heat with 3 tablespoons of the coconut oil.
Once the oil is hot and shimmering, add the mustard seeds and cumin seeds. Shake the pan back and forth or stir frequently. Once the mustard seeds start popping (it should happen pretty quickly), add the onion and a pinch of salt. Reduce the heat to medium. Cook, stirring occasionally, until the onions are nicely golden brown, 6 to 8 minutes.
Add in the garlic, ginger, and serrano if using. Cook for 1 to 2 minutes, stirring frequently. Add in the dried chiles, curry leaves and asafoetida, if using. Cook for just 30 seconds, stirring frequently.
If the oil has dried up, add 1 tablespoon more coconut oil. Add about half of the spinach and ¼ teaspoon kosher salt. Use tongs to toss the spinach and stir very frequently. Once wilted, add the second half of the spinach and stir fry until it’s all wilted.
Immediately take the spinach tadka off the heat, and pour over the warm dal, stirring in.
Taste the dal, adding another squeeze of lemon juice and season with salt as needed. Garnish the dal with cilantro. Serve with rice.