In a large pot (I like to use my Dutch oven) heat the cooking oil over medium-high heat.
Add the cumin seeds and cook for about 1 minute, until they just begin to darken.
Add the garlic, and sauté for 3-4 minutes until it has browned, but watch it very carefully so that it doesn't burn.
Add the crushed tomatoes and stir to combine, then add ginger, coriander, turmeric, salt, and cayenne.
Sauté this mixture for 5 minutes, stirring frequently.
Add the water and mung beans. Increase the heat to high and bring the mixture to a boil, then reduce to low, cover, and simmer for 30 minutes, stirring the mixture once or twice.
Taste a few mung beans to make sure they are cooked. If they are, stir in the coconut milk, and increase the heat to medium-high.
Once the curry comes to the boil turn off the heat. Stir in the cilantro and the juice of one lime. Taste and decide whether you'd like to add the second lime as well.