Heat a large pot or deep skillet over medium-high heat. Add the cumin and coriander seeds and toast until fragrant and lightly browned, about 45 seconds.
Add 1 tablespoon of water and the garlic. Sauté for about 2 minutes, adding more water as needed to prevent sticking.
Add the chopped tomatoes, ginger, and turmeric. Cook, stirring occasionally, for about 5 minutes, until the tomatoes begin to soften.
Stir in the lentils, cayenne pepper (if using), and 2 cups water. Bring to a boil.
Reduce the heat to low, cover, and simmer for 30 to 35 minutes, or until the lentils are tender. Stir occasionally to prevent the lentils from sticking to the bottom of the pot.
Once the lentils are soft, stir in the coconut milk and bring the mixture back to a gentle simmer.
Remove from the heat and stir in the cilantro.
Serve over rice or alongside naan bread.