No, not leaf miner - the leaves would be half-eaten and yellowing if it was that.

Could well be Eelworm though. "These tiny worm-like pests are too small to see with the naked eye, but the damage they do is all too easy to identify. Young plants become swollen and twisted. Young or fully grown plants infected with onion eelworm have a soft feel to them and often the bulb splits at the base. The bulbs loose all their natural crispness.

There is no cure available for this condition, prevention is the only cure. Destroy all infected plants by burning them. Do not plant onions, leeks, beans carrots or parsnips on the infected soil again for two years."

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