A Red Onion.
Red onion seeds can be obtained from Pitchfork Jeb's Farmer Supply $0.60 a seed. Each seed yields 6 red onions.
Red Onions are a ROOT Crop
Harvest with: Scythe or Root Crop Harvester
There are many factors that affect growth time. But generally Red Onions should take about 5 to 8 minutes to grow IN SEASON. Consult your Farmer's Almanac if you have questions regarding seasons.
Farming level 5 required to plant Red Onions.