Find a Wild Onion in the Grasslands and pull it out of the ground. Plant it into a deep tilled row, then water with bowl of water or full water pouch, and after 3 minutes, the onion will bloom and Onion Seeds can be taken.
An onion can be replanted into a deep tilled row in order to acquire seeds as above. Note: If you don't intend to use the seeds immediately, place them into a bowl, otherwise they will decay after 5 minutes. Individual seeds can be taken out of the bowl to plant, or the bowl of seeds can be used directly on the plot.
Wild Onion (plant)
Onion is used to make salsa.