Recently I have had great Onion crops from Sturon sets.
This is what I do, everything is prepared for a planting date of about 20th March give or take depending on the recent weather and soil temperature. I have sandy ish soil which is quite light.
Early March I till the bed over at least twice to get a fine tilth, about one week before planting a rake in all of the compost from my Dalek's, this is crucial. Last year I din't have enough compost for the complete bed and the ones that didn't get the compost were very noticeably smaller. I wait for a dry day to plant and use my index finger to dib a hole which means the set is very slightly above the soil but only just. DO NOT push the set into the ground it will damage the root structure, just drop the set in the hole your finger has created and firm the soil back around them, it takes time but it is worth it because I still have large Onions in my garage and they will see em through until April or early May. I plant in rows 18 inches apart and 9 inches between each set. I know this is generous but the extra space allows the set to grow larger and also helps when weeding.
That is it for the next two months and I do nothing to the bed apart from watering when dry and weeding regularly, not allowing any weed to take away precious nutrients from my sets. I poster on here said he weeds for the first and last 10 minutes of each Lottie visit and I follow this rule.
Around the last two weeks in May I place a small circle of Growmore around each and every set and water this in really well.
And that's it I do get fantastic Onions which are the size of large oranges or medium grapefruit, not one of them are small. My plotters either side of me have the exact same soil and their results are variable, because they don't do as I do.
So my Onion mantra is:
Space to grow.
Compost from the dalek when planting (vital)
Water regularly when dry.
Weed and weed again (I hand weed when needed to prevent damage to the bulbs)
Feed late May