How to Make It
Drain and rinse the mung beans. In a large saucepan, melt 4 tablespoons of the butter. Add the ginger and jalapeño and cook over moderately high heat until fragrant, about 2 minutes. Add the mung beans, turmeric and 1 teaspoon of salt and cook, stirring, for 1 minute. Add the 2 cups of water and bring to a boil. Simmer over moderately low heat, without stirring, until the mung beans are tender and the water has evaporated, about 20 minutes.
Meanwhile, in a medium saucepan, heat 1 inch of vegetable oil to 325°. Add half of the sliced onions and fry, stirring occasionally, until browned and crisp, about 7 minutes. Using a slotted spoon, transfer the onions to paper towels to drain. Repeat with the remaining onions.
In a small skillet, melt the remaining 1 tablespoon of butter. Add the cumin seeds and dried chiles and cook over moderately high heat until fragrant, about 30 seconds. Scrape the cumin-chile butter into the mung beans and stir in the lemon juice, cilantro and tomato; season with salt. Ladle the bean stew into bowls, top with the fried onions and serve.