Toast the cumin seeds in a dry heavy-bottomed pot over medium heat until fragrant and slightly smoky, about 30 seconds, then set aside.
Rinse the red lentils thoroughly under cold water until the water runs clear, then drain well.
Heat oil in the same pot over medium heat, then add the minced garlic and grated ginger, sautéing until fragrant and just starting to turn golden, about 1 minute.
Add the toasted cumin seeds back to the pot, then stir in chopped tomato and turmeric, cooking until the tomato softens and releases a fragrant aroma, about 3-4 minutes.
Pour in water or broth, then bring the mixture to a boil over high heat, watching for bubbles and steam to rise.
Add the rinsed lentils to the boiling liquid, then reduce the heat to low, partially cover, and let simmer gently for about 20-25 minutes, stirring occasionally, until the lentils are soft and starting to break down.
Once the lentils are tender and the mixture has thickened into a saucy consistency, stir in the chopped spinach and cook for another 2 minutes until wilted and vibrant green.
Season with salt to taste, then let the dal rest off the heat for about 5 minutes to allow flavors to meld and develop deep, soulful aromas.
Serve hot, garnished with a drizzle of oil or a squeeze of lemon if desired, alongside rice or flatbread for a complete, comforting meal.