Meaning of "at once"

[Cooliegirly]

The soft, pulpy flesh of the passion fruit possesses a flavor at once tart and sweet that has captivated many prominent chefs.

What does at once mean in here? Does it mean sometimes it's tart but most of the time sweet?

[Conchita]

Here, 'at once' means 'at the same time'. Passion fruit is both tart and sweet.