Rinse the red lentils thoroughly under cold water until the water runs clear, then set aside to drain.
Dice the onion finely and mince the garlic, preparing them for sautéing.
Heat the ghee or oil in your heavy-bottomed pot over medium heat until shimmering and fragrant, about 1-2 minutes.
Add the cumin seeds to the hot oil and toast for about 30 seconds until they crackle and release a warm aroma.
Stir in the diced onion and cook, stirring occasionally, until it turns golden and fragrant, about 3-4 minutes.
Add the minced garlic and chili powder (if using), cooking for another minute until aromatic and slightly golden.
Pour in the drained lentils along with turmeric, stirring to coat the lentils with the spices, and cook for 1 minute.
Pour in water or broth, then bring the mixture to a gentle simmer. Cover partially and cook for 20-25 minutes, stirring occasionally, until the lentils are soft and breaking apart.
During the last 2-3 minutes of cooking, stir in the chopped spinach and cook until wilted and vibrant green, about 2 minutes.
Turn off the heat, squeeze fresh lemon juice over the dal, and stir to combine, brightening the flavors.
Let the dal sit off heat for a minute or two, then taste and adjust salt if needed. Serve hot, garnished with a drizzle of oil if desired, alongside rice or flatbread.