Rinse rice and soak in around 1-cup water for 4-hours.
Rinse all dals (toor dal, chana dal and urad dal) and soak them in 1-cup water for 4-hours. (You can soak rice and all dals together. No need to soak them
Drain excess water from rice and dals and transfer them to the jar of a blender. Add 1/2 cup yogurt and salt.
Blend until smooth consistency. If required, add 1-2 tablespoons water while grinding. Don’t add more water.
Transfer the batter to a medium size container. Cover it with a lid and keep in warm place to ferment for around 8-10 hours.
When you stir the fermented batter, you should see the tiny air bubbles because of the fermentation.
Add grated bottle gourd (lauki/doodhi), ginger-green chilli paste, 1-teaspoon oil and turmeric powder.
Mix well. Batter should have thick consistency. If it is watery, add 1-2 tablespoons sooji/rava to make it thick.
Add 1/2 teaspoon baking soda
Heat 1-teaspoon oil in a small non-stick pan over medium flame. Add 1/4 teaspoon mustard seeds and when they begin to crackle, add 1/4 teaspoon cumin seeds, 1/2 teaspoon sesame seeds, a pinch of asafoetida and 2-3 curry leaves.
Pour approx. 1/2 cup batter (more or less) depending on the size of a pan and spread it evenly with spatula to make 1-inch thick handvo.
Cover the pan with a lid or a plate. Reduce flame to low and cook until top surface looks cooked as shown in the picture and bottom surface turns light brown (this will take around 4-5 minutes). Time required cook greatly depends on the thickness of handvo you are making.
Flip it gently with spatula and cover cook until another side turns golden brown, for around 3-4 minutes.
Transfer it to a plate. Follow the same process for remaining batter. Handvo is now ready for serving with tea or chutney as evening snack.