Birasta means the onion browned. This is used mostly in Mughlai cuisine to give the rich texture & color to the gravy.
Slice the onions in moon shape. Add it in hot oil till light brown. Drain & keep it on a kitchen towel. When cool it is crisp. Can be stored for a month in fridge .Use as required.
Tip: For Crispier Onion add some salt at the time of frying.