Heat the oil in a heavy-bottomed pot over medium heat until shimmering and slightly fragrant.
Add the cumin seeds and toast for about 1 minute until they start to pop and release a nutty aroma.
Stir in the minced garlic and grated ginger, cooking for another 1-2 minutes until fragrant and slightly golden.
Add the ground turmeric and coriander, stirring for 30 seconds to toast the spices and deepen their aroma.
Pour in the diced tomatoes and cook for 3-4 minutes, stirring occasionally, until the mixture thickens slightly and smells fragrant.
Add the rinsed red lentils and pour in the water or broth, stirring well to combine.
Bring the mixture to a gentle boil, then reduce the heat to low and cover the pot.
Let the curry simmer gently for 20-25 minutes, stirring occasionally and watching for the lentils to become soft and break down, creating a thick, creamy texture.
Check the consistency; if it’s too thick, stir in a splash more water and simmer a few minutes longer.
Remove from heat and stir in freshly squeezed lemon juice, tasting and adding salt as needed.
Let the curry rest for 10 minutes off the heat to allow flavors to meld and thicken slightly.
Serve hot with rice or bread, enjoying the thick, fragrant, and comforting lentil curry.