How to Prune onion 'Sturon'?
No pruning required

onion 'Sturon' · Allium cepa 'Sturon'
Grow in fertile, moist but well drained soil, in full sun, in a sheltered site. Plant out sets in March-April for a late Summer harvest or in Autumn for a Summer harvest the following year. See Vegetable Cultivation
No pruning required