Using food processor mince onion, garlic and curry leave to fine pieces. If your food processor is not working fine, then chop them manually with the knife.
In a large mixing bowl, combine all the ingredients together with a wooden ladle.
Cover the lid soak for 3 days, by stirring and mixing them once in 8 hours. On the second day, you might be noticing the mixture becomes moisture and little watery.
On the third day, Preheat the oven to the minimum setting(my oven has 170°F). Take the baking tray, and line with baking sheet(I found parchment paper was working fine for me, for the lining).
Now take a fistful and squeeze tightly and arrange them in the baking tray, and collect the excess spiced water in the same vessel. Repeat the same with the remaining onion mixture.
Bake onion balls for 3½-4 hours. Leave the excess spiced water in the vessel, by covering with the lid and keep aside. (This is a DAY-1 pic.)
Once baked bring the baked onion balls to the room temperature. Then crumble all the baked onion balls to the kept aside excess onion spiced water. Allow onion to get absorbed with spiced water for another 12-16 hour. (This is a DAY-2 pic).
The Next day repeats the earlier step, squeeze the onion mixture, and Bake the onion balls for 3½-4 hours and after baking leave them in the same shape. (On the second day, you may not be getting excess water while squeezing, but if you have, repeat the same process till the onion absorbs all the water content.)
On the third day, press the onion balls tightly further more. Now the onion balls should be reduced in their shape. (This is a Day-3 pic).
Store Onion Vadagam in the air tight container and it stays good up to a year in the room temperature.