Rinse and soak rice, chana dal, and urad dal in water for 4 hours. Drain, add to a blender with yogurt and 1/2 cup fresh water, and blend until a thick, slightly coarse batter forms.

Transfer batter to a large bowl, cover, and ferment in a warm spot for 8 to 12 hours until bubbly and tangy.

In a small pan, heat 1 tbsp oil, then temper mustard seeds, sesame seeds, and curry leaves until the seeds pop and leaves crisp. Stir the tempering into the fermented batter along with turmeric, ginger, green chilies, sugar, salt, and the grated vegetables. Right before cooking, fold in the baking soda.

Heat a heavy cast iron skillet over low heat, add 1 tbsp oil, and pour in the batter. Cover and cook for 30 minutes, then flip onto a plate, slide back into the pan with a little more oil, cover, and cook the second side for 15 to 20 minutes until crisp.

Slide the handvo onto a cutting board, let it rest 1 minute, then slice and serve hot.