Rinse the rice, chana dal, and urad dal together, then cover with plenty of warm water and soak for at least 4 hours or overnight.

Drain the soaked grains and add them to a blender with water. Blend until the mixture forms a slightly coarse batter with a texture similar to wet sand.

Transfer the batter to a large bowl, cover with a plate, and place in a warm spot—like an oven with the light on—to ferment for 8 to 10 hours until it has risen slightly and looks fluffy.

Stir the cabbage, squash, garlic, green chili, lemon juice, salt, sugar, ginger, turmeric, and red chili into the fermented batter. Allow the mixture to rest for about 30 minutes until the vegetables release their moisture and the batter loosens to a thick cake-like consistency.

Preheat the oven to 425°F and grease a 9x13-inch baking pan, lining the bottom with parchment paper.

Heat the oil in a small pot over medium-high heat. Add the mustard seeds and cook until they begin to spatter, then remove from the heat and immediately stir in the asafetida and curry leaves, covering the pot briefly if the leaves pop.

Stir half of the hot oil mixture and the Eno fruit salt into the batter, mixing thoroughly, then pour the batter into the prepared baking pan.

Pour the remaining oil mixture evenly over the top of the batter, ensuring the mustard seeds are well-distributed, and sprinkle with sesame seeds.

Bake for about 35 minutes until the edges are golden and an inserted toothpick comes out mostly clean; avoid overbaking to prevent the cake from drying out.

Switch the oven to the broiler for 1 to 2 minutes if the top needs more color, then let the handvo cool in the pan for 1 hour before slicing into squares.

Serve at room temperature or warm with green chutney or ketchup.