The welch onion is not welch at all

The Welch onion (Allium fistulosum) is an unusual onion. It doesn’t form a bulb. It is grown for its large, hollow top, and scape, multiplies readily, and is almost evergreen.

The common name "Welsh onion" does not refer to Wales but derives from a near obsolete use of "welsh" (little w, not capital W) which means "foreign, non-native", as the species is native to China, though cultivated in many places and naturalized in scattered locations in Eurasia and North America. Heavily used in Korean cuisine (called dae-pa), Chinese (called cong), and Japan (naganegi).

Nicely blanched naganegi

The species is very similar in taste and odor to the related common onion, Allium cepa, and hybrids between the two (walking onion) exist. This species, however, does not develop bulbs, and possesses hollow leaves (fistulosum means "hollow") and scapes. Larger varieties resemble the leek, such as the Japanese negi, whilst smaller varieties resemble chives. A. fistulosum can multiply by forming perennial evergreen clumps.

The very standard, large, hollow stems of Allium fistulosum

Plant as seed inside or in cooled greenhouse in summer, or get starts to plant out in September or October. This is a perennial species that you will divide as needed, and may go partially dormant in the early, dry part of summer. Grow in full to part sun, with improved garden soil. Harvest as needed.