Take fine semolina, ginger-chili paste, salt, sugar, sour yogurt and water in a large bowl. Whisk everything well until you get a smooth batter.
Cover it and let it rest for 30 minutes. During this resting time, semolina will soak up some liquid and the batter becomes thick.
When resting time is about to be done, prep the steamer. Add a glassful or two of water to the steamer and bring it to a boil. Grease the dhokla tray with oil and keep it ready.
After resting time, add oil and lemon juice. Mix well.
Divide the batter into two bowls.
Add ½ teaspoon ENO to the one bowl only. Stir and mix in one direction only. The batter will froth up and becomes light.
Immediately pour the batter into the greased tray. Sprinkle crushed black pepper over the batter.
Place the tray into the steamer and steam on medium heat for 9-10 minutes. Check the doneness by inserting a toothpick or a knife and it should come out almost clean.
Remove the tray and let it cool down for 2-3 minutes and then cut into diamond (or square) pieces using a sharp knife. Steam the remaining batter similarly by adding, and mixing the ENO.
To make the tempering, heat the oil in a small tadka pan on medium heat. Once hot add mustard seeds and let them splutter.
Add curry leaves and slit green chilies. Fry for 30 seconds and turn off the stove.
Pour the tadka over the steamed dhokla using a spoon.
Remove the dhokla using a flat spatula to a plate and serve.