common/src/main/resources/data/farmingforblockheads/recipes/market/farmersdelight does not have a file for farmersdelight:onion, only farmersdelight:wild_onions, which seems odd considering all other crops are handled from Farmers Delight from what I can see.

I suggest adding the file onion.json with the following code, which follows how the market handles rice from the same mod:

{"type":"farmingforblockheads:market","category":"farmingforblockheads:seeds","preset":"farmersdelight:seeds","result":{"item":"farmersdelight:onion"}}