Prepare batter base: Mix sooji and yogurt with ½ cup water until smooth and lump free. Cover and rest 15 minutes.
Adjust consistency: Add 2 to 4 tablespoons water to achieve thick but pourable idli batter consistency.
Add vegetables and seasoning: Fold in onion, capsicum, carrot, corn, green chilies, curry leaves, coriander, and salt. Add baking soda and mix gently once.
Heat appe pan: Heat on high, reduce to medium. Oil each cavity and sprinkle mustard and sesame seeds.
Cook: Fill cavities ¾ full. Cook 2 to 3 minutes, flip, cover and cook 2 to 3 minutes more until golden.
Serve: Remove when golden. Repeat with remaining batter. Serve hot with chutney or sambar.