Combine the washed mung beans and vegetable stock in a large saucepan.
Bring to a boil over medium-high heat, then reduce the heat to a gentle simmer.
Cover and cook, stirring occasionally, until the beans are tender. This will take about 40-50 minutes for split beans or up to an hour for whole beans.
Season with salt and pepper and keep warm.
To make the tarka, heat the oil in a small skillet over medium-high heat.
Once shimmering, add the minced garlic and ginger.
Cook, stirring constantly, for 5 to 8 minutes until they are deep golden brown and slightly crisp.
Stir in the cumin seeds, dried chiles, and optional fenugreek seeds, and continue stirring for about 30 seconds until fragrant.
Add the chopped scallion and cook for 1 minute more until softened.
Remove the soup from the heat and stir in the fresh lemon juice. You can remove the chiles for a milder flavor.
Pour the hot tarka directly into the soup pot and stir to combine, or ladle the soup into individual bowls and garnish each with a spoonful of the tarka before serving.
Don't rush the tarka! Sizzling the garlic and ginger until deeply golden is key to developing a rich, nutty flavor base.
