Well, Redfeathers says not an onion. I confess my immediate thought was garlic chives, Allium tuberosum, notoriously prolific and impossible to eradicate, flat leaf, white flower. Has a garlicky smell and taste. But those leaves in your pix look just slightly different. Might be a close relative. Unless you're certain they aren't garlic chives, it's worth looking into.
I don't suppose you still have the seed packet? (Yeah, you probably wouldn't be writing if you do).
Btw, Not to be confused with regular onion chives, Allium schoenoprasum, which grow in clumps, smell distinctly oniony, have slender tubular leaves and purple flowers.
