Heat a large pot or Dutch oven over medium heat. Add the cumin and mustard seeds. Toast them for 30 seconds, stirring them constantly.
Add the garlic, ginger, and red chili. Continue to stir them for another 30 seconds.
Mix in the tomatoes and ground spices. Break up any large pieces of tomatoes. Stir the bottom of the pot to clear it. Simmer the tomatoes for about 5 minutes.
Add 5 cups of water and clear the bottom of the pot again. Add the mung beans. Bring the pot to a boil, then reduce it to a simmer and cover the pot.
Add 5 cups of water and the mung beans. Stir them about so they are not just in one place. Bring the pot to a boil, then reduce the heat to a simmer and cover the pot.
Cook the mung beans for 30-35 minutes until they are tender.
Stir in coconut milk, chopped cilantro, and the juice of 1 lime.
Simmer for another 5 minutes. Taste and add more lime juice if desired.
Taste and add more lime juice or salt to taste.