"Handwa Flour" can be bought from any indian store, and owuld look like coarsely-ground idli-flour; it consists of chana dal, rawa and cornmeal, so I would recommend trying to get the actual flour first. If you simply cannot find it, then you can use yellow or white cornmeal. Please keep in mind that the latter has a typical corn flavor, which will dominate the flavor of the dish. 

Original Recipe copyright of Fun and Food

Ingredients
2 cups handva flour (or cornmeal, as a second option)
1/2 cup bottle gourd or zucchini
3 tsp shredded carrot
1/4 cup fenugreek leaves
1/4 cup spinach leaves (optional)
2 green chillies -finely chopped
4-5 garlic cloves - finely chopped
1/2 tsp ginger paste
3 cups buttermilk (or yogurt mixed with water)
1 tsp soda bi-carb (fruit salt)
** increase it by 1/2 tsp if you don't feel its frothy enough
1 tsp each urad and channa dal
salt - to taste
1 tsp lemon juice
1/2 tsp sugar (optional)

For Tempering
1 tsp mustard seeds
2 tsp sesame seeds
2 tbsp oil

Method
Take buttermilk in a large vessel. Add salt, soda and flour and mix well. As the mixture begins to froth, cover and set aside for 6-7 hours for fermentation.

Grate the bottle-gourd or zucchi and squeeze out all excess water. DO this after the fermentation is done, as leaving bottle-gourd in air oxidises it and makes it turn brown.

Now add all the vegetables, bottle-gourd, spices and green chillies to the flour mixture.

Heat oil in a pan, add urad & channa dals, and the mustard seeds. Allow to splutter, then pour half of it into the batter and mix thoroughly.

Steaming
Pour the mixture into a steel or aluminium container. Place in a Steam Cooker and cook like idlis, for about 25-30 mins, then check to make sure its cooked through. Remove from flame and keep it covered for another 7-10 mins. Once cooled, you can slice it into wedges.

Baking
Put in a greased oven proof deep container. Pour remaining seasoning and sesame seeds on the top. Place in a preheated oven at 350 degrees F for 10 minutes. Then reduce to 300 and bake for another 30 minutes until done. Check by inserting a skewer or toothpick, which should come out clean. Slice into wedges and serve hot.

Serve the Handvo with cilantro chutney or tamarind chutney; for me, I'll gladly have it with a cup of Masala Chai! 

For pictures, check out the Handvo recipe on my food blog.

Related Recipes:
Fluffy Khaman Dhokla
Tirangi Dhokla Sandwich
Baked Green Rava Dhokla