Clean the konrai flowers by picking off their petals and removing the reproductive parts. The flowers are delicate, so do not wash them first – clean them and then only rinse well in fresh, cool water.
Transfer to a saucepan and add just enough water to cover and allow the petals to float. Boil gently for about 5-7 minutes, or until the petals feel wilted and cooked. Switch off the heat and cool.
Now transfer this to the jar of a blender and blitz until you have a smooth paste. You can add the sugar and lime juice (if using) now, too, and blend again to dissolve and incorporate well. Don't try jaggery with this, unless you don't mind the resulting sharbat turning a lot browner.
Adjust taste—sweetness, sourness both. Mix or blitz again. This is your concentrate.
You can store this concentrate as-is, refrigerated for a week or more.
To serve, shake the concentrate well as the petal matter will have settled some.
Fill glasses with ice and pour over top, or just thin with water to taste. Aim for something light, slightly tangy and barely sweetened, not something that overpowers in any way—except maybe to dazzle with that lovely yellow color.
Note that parts of the Cassia Fistula tree are known abortifacients, so this is not to be consumed if you are trying to have a baby or are pregnant!