Place the lentils in a saucepan and add 3¾ cups of water. Bring to a boil over high heat, then reduce the heat and simmer for about 20 minutes or until the lentils are tender but still hold their shape.
While the lentils are cooking, use a mortar and pestle to grind the ginger, two-thirds of the jalapeños, and 2 garlic cloves into a smooth paste. Once the lentils are tender, stir this aromatic paste into the pot.
Add the coconut milk, turmeric, lemon juice, and a pinch of salt to the lentils. Cook gently over low heat, stirring frequently, until the coconut milk dissolves and incorporates fully into the mixture. Increase the heat and continue cooking for 5 minutes or until any excess liquid has evaporated and the lentils have a thick, creamy consistency. Taste and adjust the seasoning with additional salt if needed.
Crush the remaining garlic clove and set it aside. Melt the butter in a small frying pan over medium heat and add the mustard seeds. Watch carefully as they heat up, and as soon as they begin to pop and sizzle, immediately remove the pan from the heat. Quickly stir in the crushed garlic and the remaining chopped chile to create a fragrant tempering oil.
Transfer the cooked lentils to a serving dish and pour the hot mustard seed mixture over the top. Garnish with grated coconut and the remaining chopped jalapeños for extra heat and texture. Serve warm as a flavorful side dish or main course.