Measure and rinse the red lentils under cold water until the water runs clear, then set aside.
Heat the oil in a skillet over medium heat until it shimmers. Add the cumin seeds and toast until they crackle and release a smoky aroma, about 30 seconds.
Add the chopped onion and minced garlic to the skillet. Cook, stirring frequently, until the onion is golden and fragrant, about 2-3 minutes.
Transfer the toasted spices and aromatics to a medium-sized pot. Add the rinsed lentils, chopped tomatoes, turmeric, and ground coriander. Pour in the water or broth.
Bring the mixture to a boil over high heat, skimming off any foam that rises to the surface. Once bubbling, reduce the heat to low, cover partially, and let simmer gently for 20-25 minutes, stirring occasionally.
Check the lentils—they should be tender and starting to break down, with a creamy consistency. If needed, add a splash more water and continue simmering for a few more minutes.
Stir in the chopped fresh spinach and cook for another 2-3 minutes until wilted and vibrant green. Season with salt to taste.
Remove the pot from heat and squeeze in the juice of half a lemon. Stir well to brighten the flavors and release a fresh aroma.
Let the dal sit for 5 minutes off the heat, allowing the flavors to meld and the texture to thicken slightly.
Serve the warm, creamy lentil spinach dal in bowls, garnished with a drizzle of oil or herbs if desired. Enjoy its comforting, mindful presence.