Growing the best-tasting onions doesn’t happen by accident. Yes, you can grow onions by pretty much letting them do their own thing: you'll have onions; you'll have plenty of onions; you'll have onions suitable for most recipes. But, you will NOT have the most flavorful onions.
The first recommendation for growing the best onions is that you use onion transplants rather than starting your onions from seeds. Transplants can withstand light frost, so these will actually be one of the first vegetables you can plant in your garden.
Fertilization is vital to a good-tasting onion. Yes, onions will grow with little to no care, but feeding them, especially at the time you transplant them, will greatly enhance their flavor. In fact, studies at Texas A&M University have found that banding with phosphorous yields fantastic results. You do this by digging a trench about 4 inches deep where you will plant your onions, laying in about 1/2 cup of phosphorous-rich fertilizer, such as Bone Meal, per 10 linear feet of row. Fill in about half of the trench, plant the transplants, then fill in the remainder of the soil as you plant.
You should also side-dress with a nitrogen-rich fertilizer, like Hi-Yield Ammonium Sulphate, starting about 3 weeks after transplant, applying every 3 weeks until a month prior to harvest. You can check for progress by feeling the neck at or just above the soil level. The neck will start to feel soft; at this point you should stop fertilizing.
Water is also critical to growing onions with the best flavor. You should maintain consistent moisture throughout the season, being especially sure to water each time you fertilize. The closer it gets to harvest, the more moisture your onions will need, so monitor closely. Buying an inexpensive moisture tester will provide the most reliable information for you to determine watering requirements.
Plan ahead for the size of the bulbs your particular variety of onion will produce. You can plant closer together if you'd like to thin your onions at chive-size or as scallions, making room for the remainder of your onions to grow to full size. Otherwise, leave enough room on each side of each onion plant to allow for full-size bulb growth (and maybe a little more for good measure).
And, of course, harvesting your onions at their peak will yield the absolute best flavor. The most accurate indicator is that the tops will fall over. If your onions have bolted (flowered), don't wait for the tops to fall over: you might as well harvest if that happens. The bulbs will not grow any larger once they’ve flowered.
Flavorful onions? Yes you can!