Rinse the red lentils thoroughly under cold water until the water runs clear, then drain and set aside.
Heat the oil in a medium pot over medium heat until shimmering, then add the cumin seeds. Toast for about 30 seconds until fragrant and slightly darker in color.
Add the diced onion to the pot and sauté, stirring often, until it turns golden and soft, about 5-7 minutes. You should smell a sweet aroma as it browns.
Stir in the minced garlic and cook for about 1 minute until fragrant, being careful not to burn it.
Sprinkle in the turmeric, ground coriander, and cayenne pepper (if using). Stir well and cook for another 30 seconds until the spices release their aroma.
Pour in the water or broth and bring the mixture to a gentle boil. Reduce heat to low, partially cover, and let simmer for 20-25 minutes, stirring occasionally, until the lentils are soft and breaking down into a creamy texture.
When the lentils are tender and the mixture is slightly thickened, stir in the fresh spinach. Cook for 2-3 minutes until the greens are wilted and vibrant green.
Remove the pot from heat and stir in the lemon juice, adjusting salt if needed. The dal should smell bright and lemony, with a creamy consistency.
Let the dal rest for a couple of minutes before serving. Enjoy it hot, garnished with herbs or a drizzle of extra lemon if desired.
