Peel the onions and wash them with water. Cut them into thin slices using a sharp knife. Make sure the slices are almost equal in size to ensure even browning. You can also use a mandolin slicer or a food processor to slice the onions.
If the onions are very sharp, soak them in cold water for 15 minutes before slicing. You can also wear safety glasses while slicing.
Spread the onion slices on a dish towel and leave them on the counter for a few hours to get rid of the excess moisture.
Heat oil in a large pan over high heat. The oil should be enough to cover the onion slices. When the oil is hot, add the onion. Never add the onion in less hot oil. They will not become crispy afterward.
Fry on high heat for 8-10 minutes. Frying on high heat for the initial few minutes is important to get rid of the moisture. Now reduce the heat to low and fry until onions are golden brown. Keep stirring very frequently.
Once the onions are browned, remove them on a plate lined with paper towels and spread them on the plate. Keep in mind that onions will brown slightly more even after taking out from the oil.
Leave the Birista on the plate for 4-5 minutes. It will become crisp in this time. Store in an airtight container and freeze for upto 6 months. Use as desired.
Be very careful once the onions turn brown in color. They burn in a wink.Let them cool down completely before storing.The leftover oil can be used to make dosa, uttapam, or parathas. It has a lovely aroma of fried onions.