Soak yellow split mung beans overnight in at least 3 cups cold water for a minimum of 8 hours until beans have doubled in size — do not skip or shorten soaking time.
Drain and rinse soaked beans thoroughly before blending.
Place drained beans and plain yogurt into a blender or food processor and blend on high speed for 2–3 minutes until completely smooth and thick with no visible grain — scrape down sides as needed.
Transfer blended batter to a mixing bowl and add salt, black salt, fresh lime juice, and a pinch of sugar — mix well until evenly incorporated.
Taste and adjust seasoning — batter should be pleasantly savory with mild tang from lime and yogurt and subtle complexity from black salt.
Allow seasoned batter to rest for 10 minutes to allow flavors to meld and batter to thicken slightly.
Prepare idli cooker or steamer and grease molds generously with neutral oil or cooking spray.
Fill each mold approximately three-quarters full with batter to allow for slight expansion during steaming.
Steam for 10–12 minutes until mini idli are set firm to the touch and a toothpick inserted in center comes out clean.
Allow to cool in molds for 2–3 minutes then remove gently with a spoon or offset spatula and set aside.
While idli steam place roughly chopped onion, peeled garlic cloves, green chili, and roughly chopped tomato into a blender and blend into a completely smooth paste — set aside.
Heat 2 tablespoons neutral oil in a wide deep skillet over medium-high heat until hot and shimmering.
Add black mustard seeds and let splutter and pop fully for 20–30 seconds — do not rush this step.
Add cumin seeds immediately after and let sizzle for 15–20 seconds until fragrant.
Add blended onion garlic chili tomato paste to pan — stir immediately as it will splatter.
Add ground turmeric, ground coriander, and salt and cook over medium heat stirring regularly for 2–3 minutes until raw smell of onion and tomato disappears and paste has thickened and deepened in color.
Add steamed mini idli to the pan and stir gently to coat each piece in the spiced tadka sauce.
Cover pan with a lid and cook over low to medium heat for 5 minutes so mini idli absorb tadka flavors deeply.
Remove lid, stir gently one final time, and serve warm immediately.